MIDDLETOWN, Conn. — At least five people are dead, a dozen more injured and an unknown number missing following an explosion at the Kleen Energy power plant in this otherwise quiet college town.

The facility, which is under construction and was expected to open in June, was the scene of a massive firefighting and search and rescue operation Sunday, after a natural gas leak set off an explosion that reportedly could be felt as far away as New Haven.

At a press conference Sunday afternoon at a local middle school, Middletown Mayor Sebastian Giuliano said the accident occurred while a “blow down” test was being conducted on the site; the test — in which natural gas pipes are purged of fuel — is one of many procedures performed to determine whether the site is ready to open.

Authorities say they have ruled out terrorism but do not know what caused the explosion.

Firefighters from around Middlesex county, local and state police, and agents from the federal Bureau of Alcohol, Tobacco and Firearms and the Occupational Safety and Health Administration were on site to assist in rescue operations and the continuing investigation.

First responders were on the scene within minutes of the explosion, and worked through the night, looking for survivors. The site was blocked off to civilians and reporters, but aerial photos show the collapsed exterior of the generator building and a large debris field blackened by fire.

Middletown Deputy Fire Chief Marc Fongemie said an unknown number of people may still be trapped on the site and that the search will continue with dogs and thermal imaging equipment at least until this afternoon. State search and rescue teams used dogs to help to locate workers. Giuliano said there is no comprehensive list of employees working at the plan, since a number of separate contractors were on the site. Until a list is compiled — Fongemie said rescue teams hope to have it done by this morning — authorities will not know if anyone is still trapped inside.

Lt. Gov. Michael Fedele said the state would investigate whether construction sites should be required to keep a complete list of those working on site for use in case of an emergency. As many as 200 people may have been at the power plant when the blast occurred, and an unknown number escaped the facility unscathed, Giuliano said.

Gov. M. Jodi Rell toured the site Sunday afternoon as rescue operations were underway; earlier in the day, she activated the State Urban Rescue Team and Emergency Operations Center, which manage and conduct rescue operations from both Hartford and Middletown. The U.S. Departments of Homeland Security and Health and Human Services will provide assistance if asked by the state, Connecticut U.S. Rep. Rosa DeLauro said.

Though several area residents have reported damage to their houses from the shock wave following the explosion, authorities do not know how much damage occurred in the surrounding area. Wesleyan University’s campus in downtown Middletown, about four miles from the power plant, was not damaged, though students there reported feeling the ground shake as if there were an earthquake.

Matt Katz, a Wesleyan junior, was on a run about five miles away from the plant when the explosion occurred and said it was “so much louder than anything we’d heard before.”

In an e-mail to students, Michael Whaley, Wesleyan’s vice president for student affairs, said the university has been in touch with local authorities to offer assistance to members of the college community.

Giuliano said in the press conference that the focus remains on the tragedy of the deaths and injuries from the accident; everything else is replaceable, he said.

“You go to work in the morning and you expect to go home to your family at the end of the day,” he said. “There are a whole lot of people out there for whom that’s not going to happen.”

Esther Zuckerman contributed reporting from New Haven.

Correction: Feb. 8, 2010

An earlier version of this article misstated the name of the Occupational Safety and Health Administration.