Middletown - Investigators obtained a search and seizure warrant to enter the site of Sunday's deadly power plant explosion, which they deemed a crime scene, but have not been able to access a portion of the structure because it is so heavily damaged.

City officials said they hope the warrant will help them determine the cause of the massive explosion at the Kleen Energy Systems power plant that left five people dead, injured a dozen more and destroyed a portion of the structure.

"We have not ruled out acts of criminal negligence," said Acting City Police Chief Patrick McMahon. "Until we know that criminal negligence was not responsible, we're going to treat it as if it was. We're not saying it occurred. ... Until that search warrant has expired, it's still a crime scene."

State police from the Major Crime Squad and agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives and state fire marshal's office were at the scene, officials said.

As of Monday evening, investigators and search crews had not had access to a portion of the facility because it was deemed too dangerous to enter. Search crews were working the scene until about 2:30 a.m. Monday.

It was unclear Monday when crews would be able to enter the most heavily damaged portion of the structure. The search warrant had already been granted one extension.

Police Monday evening identified the five people killed in the blast and said the exact cause of death would be determined by the state medical examiner's office. The bodies of those killed were removed around noon Monday, officials said. The mayor said the majority of those harmed by the blast were members of the plumbers and pipefitters union.

Some union members were allowed past police road blocks leading toward the plant as they waited for the bodies of their fellow members to be removed, said Mayor Sebastian Giuliano.

City officials said they did not believe the death toll would rise, but could not be "100 percent" certain no one else was in the building at the time of the explosion.

About 100 people scheduled to be in the building Sunday were accounted for on Monday. However, officials said the search would continue until they are satisfied that anyone not scheduled to be in the building was found.

One of 12 people injured was Eric Mcleod of Norwich. A woman who answered a phone listed for him said the family did not want to talk and that he was still in the hospital.

"We're feeling very bad for the families who lost someone right now," said the woman who asked not to be identified. "We're grateful to the EMTs and the hospital right now."

A woman who answered the phone at the home of Ronald J. Crabb, of Colchester, who died in the blast, also declined to comment.

At an afternoon press conference, Giuliano and police and fire officials said the investigation thus far indicates that the origin of the explosion was outside, between two generators in the rear of the building, but they haven't identified the source.

Giuliano, who visited the site, said there was no evidence of scorch marks inside the plant near where the explosion occurred.

"It's like Chernobyl," he said, referring to the 1986 nuclear plant meltdown in the former USSR. "Vents on the roof are out of alignment ... the siding on the buildings is virtually gone. You can look right through it."

Officials said the plant partially collapsed and that welders were on site shoring up some I-beams that may have cracked. South Fire District Deputy Chief Marc Fongemie said crews were also checking on pressurized cylinders that could have been damaged and could pose a danger.

The blast, which was felt as far away as Ledyard, happened about 11:30 a.m. as crews were purging or clearing gas lines of air. Giuliano said it was not the first time gas lines were purged in the plant and that Sunday's work to purge the lines was one of the last times it was scheduled to be done before the plant went online.

Deputy Fire Marshal Al Santostefano said he was told by a construction worker that all safety protocols were in place at the time.

"Like the mayor said, something was missed or not done correctly and that's hopefully what the investigation will bring out," Santostefano said.

The U.S Chemical Safety Bureau issued a safety bulletin after a similar accident destroyed a North Carolina factory and killed four people last year. Workers were unsafely venting gas lines inside the ConAgra Foods factory in Garner, N.C. when that explosion occurred.

The safety bulletin called for five steps to be taken when purging gas lines that include evacuating non-essential personnel from the building when purging the gas lines, pumping the gas outside when practical and using detectors to determine any build up gas.

The nearly completed 620-megawatt plant is being built to produce energy primarily using natural gas, which accounts for about a fifth of the nation's electricity.

Kleen Energy Systems LLC began construction on the plant in February 2008. It had signed a deal with Connecticut Light and Power for the electricity produced by the plant, and would be one of the biggest built in New England in the last few years.

The company is run by former City Councilman William Corvo.

Energy Investors Funds, a private equity fund that indirectly owns a majority share in the power plant, said it was cooperating with authorities.

State leaders Monday called for further investigations to be done into the cause of Sunday's explosion.

Gov. M. Jodi Rell said she is assembling a panel of state agencies to identify the cause and origin of the explosion. A second panel will determine whether any changes should be made to state laws, regulations or building and fire codes.

Also, U.S. Reps. Joe Courtney, D-2nd District and Rosa DeLauro D-3rd District, said they received a commitment that a congressional hearing would take place to review what went wrong.

A date has not been set for the House Education and Labor Committee hearing. The committee, of which Courtney is a member, oversees workplace and employee safety issues.

The mayor's office set up a fund for the family members of the victims and said stress councilors and relief agencies would be made available today.

Anyone who experienced damage to their homes could contact the contractor's insurance carrier. However, city officials said Monday afternoon that they had no reports of any damaged homes.

Deputy Chief Fongemie said the responding crews simultaneously tried to put out any fires, assess the damage and treat the "walking wounded."

"It was a tough thing to see," he said. "It's one of those once in a lifetime incidents that you see and hope you never see again."

Day Staff Writers Claire Bessette and Chuck Potter contributed to this report.

Material from the Associated Press was used in this report.