Virginia Tech police and administrators struggled to explain late Monday why the campus was not locked down after a deadly shooting earlier in the day, and why students were in classrooms two hours later when a lone gunman entered a campus building and slaughtered 30 people, before turning a gun on himself.

The man responsible for murdering 32 people — the worst mass-murder shooting in American history — who carried no ID, remained unidentified late Monday, police said.

By the end of the spree, 33 people, including the gunman, were dead and authorities warned the toll could rise because several more were critically injured.

Students complained that there were no public-address announcements or other warnings on campus after the first burst of gunfire that left two dead. They said the first word they received from the university was an e-mail more than two hours into the rampage — around the time it is believed that the gunman struck again.

University President Charles Steger said authorities believed that the shooting at the dorm was a domestic dispute and mistakenly thought the gunman had fled the campus.

"We had no reason to suspect any other incident was going to occur," Steger said.

He defended the university's handling of the tragedy, saying: "We can only make decisions based on the information you had at the time. You don't have hours to reflect on it."

Steger said school officials are notifying victims' next of kin, and state police and the FBI are still investigating the various crime scenes. Campus police confirmed that the bodies of some of the victims had not yet been removed, and the process of identifying them was ongoing.

The university, meanwhile, is setting up counseling centers for students and faculty.

Steger explained that with 9,000 students living on campus and the more than 11,000 students commuting, e-mail appeared to be the best way to communicate the situation.

Steger said authorities at first believed that the shooting at the dorm was a domestic dispute and that the gunman had fled the campus. "Shock is an understatement," he said in reference to the shootings.

"The university was struck today with a tragedy of monumental proportions," Steger said during a press conference shortly after noon. "The university is shocked and horrified that this would befall our campus ... I cannot begin to convey my own personal sense of loss over this senselessness of such an incomprehensible and heinous act."

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The Web site for the campus newspaper, The Collegiate Times, reported that police recovered two 9mm handguns. That report was not yet confirmed by FOX News, and campus police would not confirm the report during the afternoon news conference.

The tragedy began unfolding at 7:15 a.m. Monday, when a 911 call came into the campus police department concerning an incident at West Ambler Johnston, a residence hall, reporting multiple shooting victims. While that investigation was under way, a second shooting was reported in Norris Hall, located at the opposite end of the 2,600-acre campus.

Steger said two people were killed in a dorm room in West Ambler Johnston in that early shooting.

A little more than two hours later, an unidentified lone gunman entered Norris Hall, a classroom building used by the Engineering Department, and methodically executed 30 people before turning a gun on himself.

Initial reports had as many as 28 people wounded and treated at area hospitals, but Steger revised that number to 15 during his late afternoon news conference.

Virginia Tech Police Chief W.R. Flinchum said the gunman killed himself. He said earlier reports about a shooting suspect in custody were not true, and confirmed that one gunman was dead.

Junior David Jenkins told FOX News he heard screaming in his dorm inside West Ambler Johnston residence hall Monday morning, but didn't know what it was. He later heard from other residents that there was a gunman in the building. Jenkins later heard of the mass shootings at Norris Hall.

"From what I heard, he chained up some of the doors so people couldn't get in and he basically was just going to every classroom trying to get in, and just started shooting inside classrooms," Jenkins said.

One of his friends was in a Norris classroom targeted by the gunman, Jenkins said.

"He was very fortunate," Jenkins said. "He said every single person in the room was shot, killed and was in the ground. He laid on the ground with everyone … he played dead and he was OK."

Flinchum confirmed that some of the Norris Hall doors were chained from the inside.

Victims were being treated at Montgomery Regional Hospital and Carilion New River Valley Medical Center in Christiansburg with gunshot wounds and other injuries.

President Bush said the "nation is shocked and saddened" by the shootings.

"Today our nation grieves with those who have lost loved ones at Virginia Tech," Bush said. "We hold the victims in our hearts, we lift them up in our prayers and we ask a loving god to comfort those who are suffering today."

The U.S. Senate and House of Representatives both held a moment of silence. Virginia Gov. Tim Kaine, who was heading for a meeting in Tokyo, Japan, for a two-week trade mission, is now returning to the United States.

"It is difficult to comprehend senseless violence on this scale," Kaine said. "I urge Virginians to keep these victims and their families in their thoughts and prayers."

Last August, the campus was closed when an escaped jail inmate allegedly killed a hospital guard and a sheriff's deputy involved in a massive manhunt. The accused gunman, William Morva, faces capital murder charges.

On April 13, the campus closed three of its academic halls after they received a letter stating that explosive devices were in the building. Classes were canceled for the remainder of the day. A bomb threat was also made against Torgerson Hall on April 2. A $5,000 reward has been offered for any information on those threats.

"For some reason, this just seemed a little different … it was more than just a sick joke someone was playing," one student told FOX News, referring to those bomb threats.

Flinchum said the idea that the bomb threats may be connected to Monday's shooting is "certainly a possibility we're exploring."

Student Daniel Smith was walking across field heading toward Norris Hall with his girlfriend when he heard yelling, and then a police officer whisked the pair off to safety in a patrol car.

"We weren't quite sure but we did see police taking out people who were heavily hurt," Smith said.

Smith, along with other students, said it was scary enough having a gunman roaming campus on the first day of classes last year, but between that, recent bomb threats and Monday's shooting, it's almost too much to take in.

"I never thought it could actually happen, at a big school like this but a small community. Growing up with Columbine and 9/11, it hits you in the heart, but I've never felt this before," said Smith, an engineering student. "I'm scared to see the list [of the dead victims] when that list comes out, because I'm bound to know some students on there … it's tearing at me. I've never had a big loss before. This is terrible."

Freshman Matthew Klim said he hasn't yet heard from one friend who he knows was in Norris during the shooting.

"We're all just really, really nervous to find out if she's still with us," Klim said. "It's really hard because coming down here, this has always been such a safe campus and following the events the first day of school with the shooting at Blacksburg, then having the two bomb threats, then this, it's all really hard to deal with."

Virginia Tech student Blake Harrison said he was on his way to class near Norris Hall when he saw chaos.

"This teacher comes flying out of Norris, he's bleeding from his arm or his shoulder ... all these students were coming out of Norris trying to take shelter in Randolph [Hall]. All these kids were freaked out," Harrison said.

The students and faculty were barricading themselves in their classrooms after what one person described as an Asian male in a vest opened fire.

The shooter was "wearing a vest covered in clips was just unloading on their door, going from classroom to classroom … They said it never seemed like it was going to stop and there was just blood all over," Harrison said.

Matt Merone, a senior, was on his way to campus Monday morning when he saw a police officer grab a male student who was bleeding from his stomach area and put him in a police vehicle, presumably en route to a hospital. Other students were seen jumping out windows to escape the gunman.

Student Amanda Johnson was walking between Norris and Randolph halls around 9:45 a.m. when she heard six shots fired.

"I've been target shooting since I was a little kid, so I knew what the sounds were," said Johnson, who saw a male student jump out of a Norris Hall window to escape.

"It just seemed like students were trying to figure out any way to get out of that building as soon as possible," added student Mike O'Brien.

Students said the first e-mail warning they got from the university about any shootings came more than two hours after the first shots were fired, around 9:30. By that time, the second shooting had taken place.

"I kind of want to know basically what happened … why school wasn't closed" after the first shootings, said freshman Kelly Kaskiw. "Lots of students are confused about that, whether the situation could have been prevented or not."

Many students didn't check their e-mail before heading to class Monday, so they didn't read the school's warnings about the first shooting. Those who did check their e-mail said they stayed put.

"There are police driving throughout the neighborhoods with a loudspeaker saying, 'This is an emergency, everyone stay inside, we're looking for suspicious activity," said Brittany Sammon, a senior Virginia Tech student staying at an apartment off campus. "There's no one outside at all, there's no traffic, there's nothing … everyone's doing what they said."

Premeditated Murder?

The FBI joined police on the scene to investigate. Agency spokesman Richard Kolko in Washington said there was no immediate evidence to suggest it was a terrorist attack, "but all avenues will be explored."

A senior official with the Bureau of Alcohol, Tobacco, Firearms and Explosives told FOX News that the agency's response to the Virginia Tech incident was "immediate" and the bureau is making available all of its local and national resources, including its crimes lab, to the Virginia State Police.

Ten ATF agents went to the Virginia Tech campus to assist with weapons identification. They were collecting shell casings and running some preliminary tests on scene.

Once the weapon has been identified, they will begin an "urgent trace" to determine its origins — where it came from, to whom it was registered and its history of ownership. All material will be sent to the ATF's national crime lab in Maryland.

The ATF is also assisting with "forensic mapping" of the crime scene — a painstaking process employed by investigators that 'maps out' the scene and incident in minute detail.

Former Assistant FBI Director Bill Gavin said if reports that the shooter chained the doors to Norris Hall are true, that is "definite proof of premeditation," as is the number of magazines and rounds of ammunition he apparently had.

"He didn't take that just to shoot one particular person," Gavin said. "He had to have something going on there that said he was going to shoot a whole bunch of people at the same time."

All classes were canceled for Monday and Tuesday but campus will open at 8 a.m. EDT Tuesday. Faculty and staff on certain parts of campus were told to go home.

Families wishing to reunite with students are suggested to meet at the Inn at Virginia Tech. School officials are making plans for convocation Tuesday at noon at Cassell Coliseum.

FOX News' Ian McCaleb and The Associated Press contributed to this report.