Leaking natural gas caused an explosion that injured 14 construction workers and damaged four floors of an unfinished 30-story hotel, authorities said Tuesday.

The blast at the bayside Hilton Monday afternoon was an apparent accident, caused by a gas leak in a fifth-floor utility room, said San Diego Fire-Rescue spokesman Maurice Luque. Investigators had not yet determined what ignited the gas.

"We're not sure what it was, but it could have been electrical or the flames from the boilers," Luque said.

Three workers who suffered second- and third-degree burns, one over 35 percent of his body, were in medically induced comas at UCSD Medical Center. A fourth was stable and in good condition, said hospital spokeswoman Kimberly Edwards. Six other workers were released Monday night.

• Click Here for Photos of Blast-Damaged Hotel

One worker remained in fair condition at Scripps Mercy Hospital and three others were released, said hospital spokeswoman Kristin Reinhardt.

Doctors said injured workers described a bright light and brief, intense heat consistent with a gas explosion.

Investigators from the state Division of Occupational Safety and Health were questioning workers to determine what happened in the moments before the blast, which filleted the sides of large metal boilers and sent glass and facade material showering onto a hotel driveway below.

Witnesses said they heard a loud blast and saw a puff of smoke blow out the side of the hotel.

"It shook the whole building like a bomb," said Matt McBride, the general manager of the Tin Fish bar, a restaurant about 150 yards from the hotel. McBride was setting up an outdoor patio Monday afternoon when the sudden blast cracked just after 2 p.m.

McBride said he and other bar employees froze as they saw smoke pour out of the building.

"The reverb was what scared us," McBride said. "Everybody was saying bomb, bomb, bomb, terrorist, terrorist, terrorist."

Bruce Ragland, who runs an entry gate at the Petco Park baseball stadium about 200 yards from the hotel, said "it was just like watching a Bruce Willis movie, like you know, 'Die Hard."'

Ragland said he was looking right at the building when the explosion blew through two sides and set off a shock wave.

The building did not burn.

The blast happened in a "mechanical area" of the building containing gas, electric and other utilities, said Deputy Fire-Rescue Chief Perry Peake.

The blast damaged floors four through seven of the building, Mayor Jerry Sanders said.

More than 400 construction workers were at the site at the time of the blast Monday afternoon, Sanders said.

The site was surrounded by firefighters and utility crews, and a truck with the markings of the Bureau of Alcohol, Tobacco, Firearms and Explosives arrived.

The 30-story Hilton San Diego Bayfront hotel was scheduled to open in December, according to Hilton's Web site. It will have 1,190 rooms and more than 165,000 square feet of meeting space.

Hilton Hotels Corp. has not taken possession of the hotel from the builder, Hensel Phelps Construction Co., said Karima Zaki, the hotel's vice president for new development. She said it was too early to know whether the hotel will open on schedule.

Hensel Phelps officials declined to comment on questions sent by e-mail.

As scheduled, the Padres baseball team hosted the St. Louis Cardinals Monday night at Petco Park.