Huddled beneath a desk, a bookshelf shoved against a locked office door, Regina Kuruppu held tightly to her co-workers' hands and began to pray aloud, unable to drown out the terrifying cries coming from one story below. "Heavenly Father," she said as her colleagues tried to offer words of comfort. "Watch over my family. Watch over us."

When the fire alarm had sounded minutes earlier, Kuruppu was sitting at her desk on the second floor of Building 3 at the Inland Regional Center. A 19-year veteran of the organization that helps those with developmental disabilities such as autism and cerebral palsy, Kuruppu was wrapping up her pre-vacation emails, sending notes of thanks to the center's supporters. By week's end she would be off for the month and home doing what she loved: baking for the holidays and stringing Christmas lights outside her home.

The alarm, she thought, was just a drill — until she got downstairs and saw two bodies in a pool of blood by the door of the auditorium, the victims' eyes open and unblinking.

She didn't stop to wonder what evil had come to the center that morning or why. Those questions would come later, for her and for a nation left grappling not only with another mass shooting, but another potential act of terrorism at home.

Barricaded back upstairs, Kuruppu could only pray for protection — even as she prepared to die. She thought of her son. She texted her sister. They exchanged their "I love yous." A woman of strong faith, Kuruppu tried to hold fast to hope. But she kept seeing the unseeing eyes downstairs.

"I'm going to leave this world," she thought, believing this December day just might be her last.

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Down the street from a Sizzler, around the corner from an IHOP, the Inland Regional Center sits in a nondescript office complex in the middle of Anytown, USA. For San Bernardino — a hardscrabble city that has been crippled by economic woes, whose citizens have grown used to adversity — the facility has always been both a reliable employer for some 600 area residents and a haven for the tens of thousands of vulnerable clients it serves.

December is always an especially busy month at the center's popular auditorium space on the first floor of Building 3, and Kuruppu's co-workers had spent the day Monday decking out the room for upcoming festivities. A tree wrapped with red ribbon sat off to one side. Ornaments hung from the ceiling. Long banquet tables were adorned with red cloths.

The center's annual holiday party for clients was Tuesday afternoon, and a winter dance complete with ugly sweater contest was scheduled three days later. In between, as so often happens, the auditorium had been rented to an outside client: The county's Division of Environmental Health Services had scheduled a training session and holiday luncheon for as many as 90 employees on Wednesday.

Food inspector Chris Nwadike arrived at the auditorium that morning looking forward to time away from the daily grind of the Health Department, where he'd worked some 25 years. Doughnuts, tamales and some downtime with co-workers awaited at an event that always mixed business with pleasure.

For Bennetta Betbadal, a fellow food inspector, the gathering was a chance to see old friends, and she was excited to make a presentation to supervisors and colleagues. For Jennifer Stevens, who started work at the health agency after graduating from college in June, the luncheon was an inaugural event.

As the attendees settled in, supervisors reviewed the year's work and announced plans to hire more employees in 2016. They played a trivia game, vying for gift cards. Some videos followed, including a training movie about landfills, and then came a 20-minute break before lunch.

Nwadike shared a table with restaurant inspector Syed Rizwan Farook, a quiet man employed at the agency for five years who had always done good work. "He's serious with his job," Nwadike said later. "He doesn't play around." It had been a big year for the 28-year-old Farook. Co-workers threw him a shower before his daughter was born. They organized a potluck and collected cash for the new father and his bride of a year, Tashfeen Malik, a woman in Saudi Arabia whom Farook told colleagues he'd met online.

At the banquet, Farook had slipped out before the trivia game, leaving behind a jacket and other belongings, as if he planned to return.

By break time, he was still missing. Nwadike headed for the restroom, while others stepped outside for fresh air or a smoke. Environmental health specialist Denise Peraza used the time to grade papers for a class she taught. It was just before 11 a.m.

Suddenly, doors to the conference room leading to an adjacent parking lot burst open. Sunlight blazed into the room, followed by gunfire blasts. "Five rounds heard," police dispatch recorded at 10:58 a.m. Then one minute later: "Heard about 20 to 25 rounds."

Two figures dressed in black were firing semi-automatic rifles.

Stevens, 22, at first thought someone was playing a joke — until she got hit. "She looked down and she had a big hole in her side," said her mother, Lisa, who recalled her phone ringing a short time after the rampage began and hearing her daughter's horrifying words: "Mommy, I've been shot."

Peraza dove on the floor under a table, alongside co-worker Shannon Johnson, who wrapped his arm tightly around her as they tried to shield themselves with a fallen chair. "I got you," Johnson told Peraza, who felt something hit her lower back and then a hot sensation.

Nwadike had been in the men's room about four minutes when he heard a loud blast. He thought something had fallen on the roof or crashed outside. Then a spasm of gunfire tore tiles off the bathroom wall, striking a colleague who sensed immediately what was happening: "Somebody is shooting! Lie down! Lie down!"

From inside the office upstairs, Kuruppu and the three co-workers hiding with her heard the shots and then the wails and then feet running down hallways. She continued to pray as she texted her mom, her niece, her sister. "Gina stay low," her mother pleaded. "I love you," her sister said, and then she typed the words once more.

Downstairs, Nwadike took cover inside a bathroom stall, helpless as he lay listening to his friends crying in pain.

As the bullets kept coming, he, too, began to pray, all the while haunted by a single thought: "When will it be my turn?"

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At 11 a.m., San Bernardino Police Lt. Mike Madden was on his way to lunch when the call went out. At 11:03, he was outside the Inland Regional Center, assembling three other law officers to head inside. A mere 10 miles away, the police SWAT team happened to be undergoing active shooter training when a lieutenant heard over the radio: "Shots fired. Multiple victims hit." Already in gear, unit members swapped their ammo for live rounds and rolled out.

At 11:10, when SWAT member Ryan Starling arrived on site, people were running from the building. Madden and the other officers already were inside. Starling's unit headed around back. Six people were down, three of them dead. Starling guessed some of the wounded had crawled to safety. Starling, Madden — even these seasoned lawmen — were overcome by what they encountered next. Inside the auditorium, the smell of gun powder was still fresh. Water from the fire sprinklers sprayed down on a sea of carnage.

"It started flowing outside and I just remember the trail, like a little river of blood coming out," Starling said.

"Unspeakable," said Madden. "It just seemed so senseless that here's people going into their holiday festivities — and now we were dealing with death."

Scattered about, 21 people were wounded but alive, shot in the back, in the head, in the stomach. Stevens and Peraza were among them. Fourteen others were killed, including Bennetta Betbadal, who had fled her native Iran at 18 to escape the persecution of Christians after the Iranian Revolution. Shannon Johnson, whom Peraza believes kept his promise and likely saved her life, was gone, too.

As the hundreds of Inland Regional Center employees spilled from the building, Kuruppu and her colleagues stayed tucked away in the office upstairs. Maybe 20 minutes passed before the door was busted down and men in uniform stood before them, extending their hands to help.

"You're going to be OK," they promised, but Kuruppu wasn't so sure.

Kuruppu came to the center in her late 20s as a case manager with a special connection to those she counseled: Born with Treacher Collins syndrome, a rare disease that left her without cheekbones, ears or a chin, Kuruppu underwent 20 reconstructive surgeries by her teens and endured constant bullying in school. She always knew that as an adult she wanted to help others who were different, and teach them how to survive as her parents had taught her. Some two decades later, she'd worked her way up to a position as a family support specialist, serving as a liaison to other agencies, overseeing support group meetings for mothers and helping to coordinate events in the auditorium.

Through her life, she had survived a lot. Several years earlier, Kuruppu was held at gunpoint during a robbery at a fast-food restaurant. The assailant fled, and she was unharmed, but the emotional wounds that had begun to heal were ripped open by Wednesday's violence.

Later that day, she reunited with her family and, back at home, her brother hung the Christmas lights that very night to try and comfort her. That helped, some. And the strength that has always seen her though life is still there.

"I look back at what I have gone through, and there has never been a traumatic incident that I have not come back from. I will bounce back from this," she said the next day. "I hug my 23-year-old son and tell him, 'This is why I tell you I love you every day, because you never know from one moment to the next what's going to happen.'

"Fear is not what God wants us to feel. He wants us to feel at peace."

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Peace hasn't found San Bernardino yet. Too many questions remain unanswered, even as new and startling revelations begin to surface about the assailants — Farook and his wife, Malik. The two fled within minutes of the shooting, and some four hours later police found them in an SUV that had been spotted leaving the crime. A chase ended on a residential street about 2 miles away. Farook and Malik, 29, died in a shootout with police, leaving behind their 6-month-old daughter.

The couple were armed with assault rifles and semi-automatic handguns and unleashed 76 rounds until they were outgunned by nearly two dozen officers who fired 380 rounds, police said. Investigators would later find a cache of ammunition and pipe bombs and, eventually, some hints at a possible motive.

The FBI said Friday that it is investigating the shooting as an act of terrorism, but the agency's director said there is no indication that the two were part of a larger plot or members of a terror cell. A U.S. law enforcement official told The Associated Press that Malik, under a Facebook alias, had pledged allegiance to the Islamic State group and its leader. A Facebook official said Malik praised Islamic State in a post at 11 a.m. Wednesday, around the time the couple stormed the Inland Regional Center.

If the investigation confirms the massacre was inspired by Islamic extremism, it would be the deadliest such terrorist attack on U.S. soil since Sept. 11, 2001.

For those who worked alongside Farook, the allegations are difficult to fathom.

"I said, "No, it's not him! He's quiet. He doesn't make any trouble," Nwadike said. "I didn't see anything that this guy would do this type of thing. . Syed has never, ever gotten into a confrontation with anybody."

And, yet, most of the dead and wounded were county employees, many of whom had worked alongside their killer.

"When I look at those that died, I know them. I'd see them almost every morning. That touches me, and then I start thinking about myself," said Nwadike, who understands how lucky he was. The 63-year-old father of four escaped unhurt. "I'm still talking. Right now, I'm home. That's what I know."

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Arrillaga reported from Phoenix. Contributing to this report were AP reporters Gillian Flaccus, Amanda Lee Myers, Nicole Evatt and Amy Taxin in San Bernardino and John Rogers, Andrew Dalton and Christine Armario in Los Angeles.