A major highway tunnel that carries traffic under Boston Harbor to the airport reopened to buses Friday morning, a day after the governor ordered it shut down to fix two slipping bolts in a heavy ceiling panel.

The tunnels throughout Boston's Big Dig highway system have been heavily scrutinized since a motorist was killed last week by 12 tons of falling concrete ceiling panels.

Inspectors discovered more than 1,100 suspect bolts in the ceilings of that tunnel and a tunnel ramp. Gov. Mitt Romney ordered the nearby Ted Williams Tunnel's eastbound lanes closed on Thursday after state engineers discovered two bolts on a single ceiling panel there appeared to have slipped a half-inch and 1 inch.

Engineers worked through the night to reinforce the bolt systems and the tunnel reopened about 7 a.m. Friday.

"It is perhaps an overreaction, but we want to err on the side of public safety," Romney said in announcing the shutdown.

The Ted Williams Tunnel's ceiling panels are suspended using the same threaded, epoxy bolts as the panels that collapsed, but the Ted Williams Tunnel panels are lighter and the system that suspends them is considered more substantial.

Amid the inspections, Romney, a Republican considering a run for president, has started proceedings to oust the head of the Turnpike Authority, which oversees the highway system. Attorney General Tom Reilly has also launched a criminal investigation into the construction to determine if involuntary manslaughter charges are warranted.

The $14.6 billion Big Dig buried the old elevated Central Artery under Boston starting in the early 1990s. Although it's been considered an engineering marvel, the most expensive highway project in U.S. history also has also been plagued by leaks, falling debris, cost overruns, delays and problems linked to faulty construction.