Gov. Mitt Romney said Sunday it would take months to fix problems in the entire Big Dig highway system and reopen the roads, and another ramp was closed to traffic because of what he called a “systemic failure.”

The work in the tunnel closed Sunday, a three-quarters of a mile long ramp, is expected to last at least several days and comes nearly a week after the collapse that crushed a car carrying Milena Del Valle, 38.

The closure was ordered because of potential problems with 40 of the bolts that hold up the heavy ceiling panels, Romney said.

Testing on the bolts has shown that “what happened last week was not an anomaly but a systemic failure,” Romney said.

“It appears the problem is far more substantial,” than at first thought, he said.

Since Del Valle’s death July 10, motorists had been using the now-closed ramp as a detour around the accident scene.

The ramp, which connects Interstate 90 westbound to Interstate 93 north and south, had been previously identified by Romney’s inspection teams as a potential trouble spot, said Jon Carlisle, a state Highway Department spokesman.

“We’re putting additional connections between the roof and the ceiling panels,” he said, adding that the specific number of repair spots was unclear. “We’re still working on the engineering.”

Romney said Sunday’s closure was not called for because of any imminent danger. “We’re just not willing to risk people’s lives,” Romney said.

Twelve tons of concrete ceiling panels crushed the passenger side of the car being driven by Del Valle’s husband, Angel Del Valle, as they headed to Logan International Airport. Connector tunnels in both directions have been closed since then.

State and federal investigators have focused on bolts used to hold the drop-ceiling system in place. Each of the concrete slabs suspended above the roadway weighs three tons.

Romney has said there are 84 potential trouble spots in the eastbound connector tunnel where Del Valle was killed. In two other adjoining sections of the tunnel, as well as traffic ramps, there are another 278 possible problems, he has said.

In some cases, inspectors have found ceiling bolts pulled as much as three-eighths of an inch away from the tunnel’s concrete roof, he said. Investigators are focusing on the bolts and the epoxy glue used to secure them.

The $14.6 billion Big Dig buried the old elevated Central Artery that used to slice through the city, replacing it with a series of tunnels. 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.