SPRINGFIELD, Mass. – It’s a busy stretch of highway, carrying commuters, truckers and sightseers northward from Connecticut into the Pioneer Valley.

But during the past few months, Interstate 91 has seen a series of incidents that have mystified motorists and law enforcement officials. About a dozen drivers have reported their car windows were shattered by unidentified objects that police described only as “projectiles.” So far, no one has been injured, and whatever hit the cars hasn’t caused any accidents.

Most of the incidents occurred on a stretch of I-91 through Springfield and Longmeadow, where road crews are doing construction work that some officials say could increase the likelihood of debris kicking up and striking cars.

But police searches of wooded areas alongside the highway have turned up at least one suspicious clue: dozens of BB pellets.

Whether the blame lies with a BB gun-toting prankster, or with airborne pebbles, officials say they’re certain that the damage is not the work of a sniper with a firearm.

“If it’s a bullet, it’s going to remain in the car or go out the other window,” said Hampden County District Attorney William Bennett, who is investigating the reports of broken windows. “No one is reporting hearing gunshots. We’re not dealing with a firearm. We’re not quite sure what we’re dealing with.”

The reports started in April after a truck driver said his windows were broken.

as he drove along the Massachusetts-Connecticut border. In May, about a half dozen drivers said their windows were damaged, most in and around Longmeadow, just south of Springfield.

But police also heard from drivers saying their cars were struck along Interstate 391 near Chicopee and farther south on I-91 in Enfield, Conn.

The latest incidents were reported on Tuesday evening and Wednesday afternoon. All happened on I-91 in the same general area as the previous incidents and were “consistent with a low-velocity projectile like a pellet gun or paintball gun,” Bennett said.

“We had just got on the highway and the window blew out,” said Gerald Adams, who said something smashed through the rear driver’s side window of his Honda Accord earlier this month on I-391 after he left his job at a television station in Chicopee. “The glass came into the window and hit me in the back of the head.

“I’m relatively sure it’s not an accident,” he said. “There’s somebody out there doing it.”

Colleen Fenton agrees.

On her way to work at a Walgreens in Agawam this week, the 23-year-old heard five “bangs” from the side of the road as she was driving on local roads through Longmeadow.

When she got out of her car, there were five dings on the doors of her Chevrolet Cavalier.

“Nothing serious came of it, but if you hit the right person’s car the right way, you don’t know what will happen,” she said.

If it’s a shooter with a BB gun, Bennett said he’s confident police will apprehend the person.

State police have stepped up patrols of roads where windows have been breaking, and a helicopter has been surveying the area from above.

And for now, the DA says people shouldn’t be looking for different routes around the area.

“We have thousands and thousands of cars that travel on that portion of Interstate 91 every day without incident,” he said.