A manhole cover at the edge of the Thomas P. O’Neill Jr. Tunnel came loose and flew into the air during the Friday morning commute, smashing through a car’s front windshield and killing the driver, an art teacher who was headed to her job at a Milton elementary school.
Caitlin Clavette, 35, was driving south on the Southeast Expressway in Boston just before 8 a.m. when the manhole cover, which weighed more than 200 pounds, crashed into her black Honda HR-V, traveled through the vehicle, and exited the rear of the car.
The stunning fatality raised unsettling questions about motorists’ safety and prompted the state to immediately inspect 500 covers for manholes, drainage systems, and electrical panels on area highways. The accident occurred on a stretch of highway traveled by some 100,000 drivers a day.
Crews clearing the crash appeared visibly upset, said Cathi Porreca-Leonard, who passed the accident scene Friday morning.
“They were consoling each other,” she said. “It could have been anybody.”
State officials were unable to explain what may have caused the cover to come loose from the street level framework where it normally would have rested, and were investigating how it happened. They would not say whether the emergency inspections had uncovered other problems.
“Our sympathy goes out to the family of the victim involved in this horrific incident this morning,” said Thomas Tinlin, the state’s highway administrator. “This tragedy is leading us to take several steps immediately out of an abundance of caution.”
The State Police, who are investigating, did not identify the driver who was struck. But in a letter to parents Friday afternoon, Milton Public Schools officials said Clavette’s death was “a tremendous loss.”
“She was a talented and valued member of our faculty,” wrote the principal of Glover Elementary School, Sheila Kukstis, in a message to the school community. “Our thoughts and prayers are with her family and friends.”
Clavette graduated from Winchester High School and received her bachelor’s degree in art from the College of William and Mary in Virginia, according to Milton school officials. She held a master’s degree from the School of the Museum of Fine Arts in Boston.
The manhole cover that went airborne covered a storm drain under the far left travel lane of the O’Neill Tunnel, where Clavette was traveling, said David Procopio, a State Police spokesman.
The cover blasted through the windshield of Clavette’s vehicle on the driver’s side and came to a rest just outside the tunnel, Procopio said.
Her car continued for about 100 feet before stopping when it scraped a Jersey barrier between the Southeast Expressway and a connector ramp, he said.
Porreca-Leonard said she saw the damaged vehicle as she was driving home from Logan International Airport with her husband.
“The whole back of the car was blown out. It was just an awful, awful sight,” she said. “I know a lot of people have that visual now.”
Investigators will examine the manhole cover to determine whether it had excessive wear or any other flaws that could have caused it to become airborne, Procopio said.
“We are also in the process of seeing how it fits into the hole it came from, to see if the fit is satisfactory or if it’s loose,” he said.
In many cases, manhole covers are not attached to their frames with a bolt or other fastener, but instead lie flat and stay in place from their own weight.
Marc Breakstone, a Boston lawyer who has litigated personal injury cases involving manholes, said if the cover was not sitting flush to the road, it might teeter in its frame and damage it.
“I’m guessing the rear wheel or wheels of a heavy vehicle passed over the manhole, catching the exposed lip of the cover, and from the traction of the tire dislodged and caused the cover to become a projectile which flew through the passenger compartment of this poor woman’s car,” he said.
Robert Norton, another lawyer who has investigated similar accidents, agreed, noting it wouldn’t have been Clavette’s car that caused the manhole cover to go airborne.
“There’s no way that can go airborne without a vehicle going over it, and it was probably a truck,” he said. “This poor lady — she didn’t dislodge it.”
Investigators plan to review surveillance video to determine whether the manhole cover was dislodged by another vehicle before it struck Clavette’s car.
State Secretary of Transportation Stephanie Pollack described the fatal incident as a “very rare event.”
“They’re pretty hard to dislodge and even harder to get airborne,” she told reporters at the State House.
The manhole cover was last inspected in June 2014, officials said. At the time of the inspection, it “rated very good,” Pollack said.
The Transportation Department conducts inspections of hardware on the roads in the tunnel system every two years.
“There does not seem to have been any recent work that would have involved moving that manhole cover,” Pollack said.
The state had not received any prior complaints about the cover, said Jacquelyn Goddard, spokeswoman for the Transportation Department.
Pollack said she was not aware of any connection between the cold temperatures and the cover going airborne, but she said investigators would look into it. The temperature was 10 degrees in Boston at 8 a.m. Friday, according to the National Weather Service in Taunton.
In 2007, transportation officials inspected 86,000 storm grates and manholes after a software engineer from Easton was severely injured when a 250-pound storm grate slammed through his windshield on Route 128 in Westwood.
The family of Pawel Swierczynski sued, contending the grate was defective.
The lawsuit was settled before going to trial in 2010, said Norton, the lawyer who represented Swierczynski.
The terms of the agreement are confidential, he said.
“This is a sad day for us as this tragic incident revives difficult memories. We can only offer our condolences and hope they find the strength to get through this difficult time,’’ the Swierczynski family said in a statement.
In Milton, officials said Clavette began teaching art in the town’s elementary schools in 2011.
The Glover School plans to open Saturday for parents to receive guidance on discussing Clavette’s death with their children, the district said.
Christine Allen, whose 11-year-old daughter, Grace, attends Glover, said she was badly shaken by the news.
Grace had been in Clavette’s art classes for years and adored her, Allen said. This year, Grace was in a special art program during lunch on Fridays. On this Friday, the class had planned to paint tribal masks.
“She decided that she wanted to be an art teacher, too,” Allen said.
Allen said she planned to tell her daughter that what happened to Clavette was a random tragedy. “There was nothing she could have done,” she said.John R. Ellement, Steve Annear, and Joshua Miller of the Globe staff contributed. Laura Crimaldi can be reached at email@example.com. Follow her on Twitter @lauracrimaldi. Nestor Ramos can be reached at firstname.lastname@example.org. Follow him on Twitter @NestorARamos.