Boston ? The state’s next top prosecutor said Thursday that it may be hard to bring charges in the deadly collapse of a Big Dig tunnel ceiling, despite the current attorney general’s vows to hold someone accountable.

Attorney General Tom Reilly, who leaves office next month, has said he would try to pursue charges up to manslaughter in the accident.

But his replacement, Martha Coakley, said it could be difficult to bring criminal charges in a case that involves multiple corporations involved in the design, construction and oversight of the $14.6 billion Big Dig, the most expensive highway project in U.S. history.

“This is a situation where everybody is responsible for this so maybe nobody is responsible for this,” Coakley said.

The July 10 collapse of several 2-ton concrete ceiling panels from the Interstate 90 connector tunnel killed Milena Del Valle, 39, as she and her husband drove toward Logan Airport.