Boston ? At least 12 tons of concrete collapsed onto a car in a Big Dig tunnel, fatally crushing a passenger and prompting renewed scrutiny Tuesday of a troubled highway project that is already the costliest in U.S. history. The state attorney general said he plans to treat the site as a crime scene that could lead to negligent homicide charges.

The attorney general’s office already has begun issuing subpoenas to those involved in the design, manufacturing, testing, construction and oversight of the panels and tunnel.

“What we are looking at is anyone who had anything to do with what happened last night,” Massachusetts Atty. Gen. Tom Reilly said. “No one is going to be spared.”

The collapse happened around 11 p.m. Monday, when the underground highways were relatively free of traffic. About 200,000 vehicles a day travel along the roadways that make up the Big Dig, the $14.6 billion highway construction project that buried the central artery underneath the city.

Firefighters inspect a collapsed section of ceiling in the Interstate 90 tunnel eastbound through Boston that fell Monday night and killed a woman riding in a car.

Four concrete slabs weighing three tons pancaked down onto the passing Buick sedan, killing Milena Del Valle, who was riding on the passenger side, which bore the brunt of the collapse. Her husband managed to crawl to safety through a window that was only a foot wide. He suffered only minor injuries.

Debris shut down part of Interstate 90, backing up traffic for miles Tuesday. Authorities hoped to reopen it today, but they were still removing about 30 ceiling slabs from the accident site and checking at least 17 other areas with similar “tiebacks” holding ceiling panels in place.

Gov. Mitt Romney pinned much of the blame on the head of the Massachusetts Turnpike Authority and promised to take legal action to oust Matthew Amorello.

“People should not have to drive through the Turnpike tunnels with their fingers crossed,” said Romney, a longtime critic of Amorello.

The trouble-plagued Big Dig has gained nationwide notoriety for rising costs, years of traffic snarls, a criminal investigation into faulty concrete from suppliers and problems with hundreds of leaks that sprouted in another of the Big Dig tunnels.

Amorello said the tunnels are safe and said he would not step down. “We will work on all of this, together, cooperatively,” he said.

Modern Continental, the contractor of that portion of the project, issued a statement that its work “fully complied with the plans and specifications provided by the Central Artery Tunnel Project.”