SAN DIEGO — A mechanical failure or gas likely triggered an explosion that rocked a waterfront hotel under construction, damaging four floors and injuring 14 workers, five critically, a fire official said.

The blast occurred around 2 p.m. Monday, ripping off part of the Hilton hotel's facade and sending debris flying. More than 400 construction workers were at the site at the time, Mayor Jerry Sanders said.

Bruce Ragland, who runs an entry gate at the Petco Park baseball stadium about 200 yards from the hotel, said he was looking right at the building when the explosion blew through two sides and set off a shockwave.

"It was just like watching a Bruce Willis movie, like you know, 'Die Hard,"' Ragland said.

San Diego Deputy Fire-Rescue Chief Perry Peake said the explosion occurred in an area of the building containing gas, electric and other utilities and appeared to have been caused by a mechanical failure or gas.

"We want to rule out all possibilities before making an official report, but we're pretty confident that's how it's going to come out," he said.

Matt McBride, general manager of the Tin Fish bar about 150 yards away, said he was setting up the outdoor patio bar when the blast occurred. He said he and other workers froze as they saw smoke come out of the building but no flames.

"It looked like a bomb," McBride said. "It shook the whole building like a bomb. The reverb was what scared us. Everybody was saying bomb, bomb, bomb, terrorist, terrorist, terrorist."

The hotel near the San Diego Convention Center showed serious damage, and debris littered a driveway beneath one corner of the structure. Facade material dangled from the structure, drapes dangled out windows and a large equipment room with tanks and pipes was left exposed.

The building, however, did not burn.

Three of the five workers in critical condition were in a hospital burn unit, fire spokesman Maurice Luque said. Other workers who were also burned or struck by debris suffered injuries ranging from serious to minor, authorities said.

Ten people were taken to UCSD Medical Center, said Dr. Irving Jacoby, attending emergency room physician. Three were in the burn unit, one with burns over 35 percent of his body.

"They talked about a bright light and an intense heat," Dr. Bruce Potenza, of the hospital's burn unit, said of the injured.

By late Monday, six of the 10 at UCSD had been released, said hospital spokeswoman Kimberly Edwards. Another was stable and in good condition, she said. The three burned victims were in induced comas, Edwards said.

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 say whether the hotel would open on schedule.

Hensel Phelps officials did not immediately respond to questions sent by e-mail.

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Associated Press Writers Elliot Spagat and Bernie Wilson contributed to this report.