SACO, Maine (AP) — Police in Saco, Maine, asked residents to shelter in place Friday afternoon while they searched for suspects after an exchange of gunfire and vehicle crash at a busy intersection spurred a brief citywide school lockdown.

The reports of gunshots came in at about the same time as the multi-vehicle crash downtown, police said. It involved a school bus that had children on board, but none were hurt, city spokesperson Emily Roy said.

A police spokesperson described the investigation as “an ongoing, fluid situation.” Police said they were searching for four people who fled from one of the cars involved in the crash, and that one of the people appeared to have been shot. They also said they recovered a firearm from one of the vehicles involved in the crash.

Roy said the crash involved the school bus from nearby Old Orchard Beach and another vehicle.

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“There were reports of gunfire downtown and then reports of a vehicle crash involving a bus from Old Orchard Beach,” Roy said. “Right now there is police activity due to the ongoing incident.”

Ryan Guay, a supervisory Deputy U.S. Marshal in Portland, Maine, said the U.S. Marshals Service was on the scene helping the Saco Police Department. He said there was some type of motor vehicle crash and that the suspects fled the scene and dispersed into the Saco area.

The situation sparked a large police presence in downtown Saco, with members of several police departments responding.

Thornton Academy, a school located near downtown, said in a Friday afternoon statement that it had entered “full lockdown” and that “students remain safe” while police monitored the situation. Students were later released.

“Students who drove to school have been dismissed,” the school said in a statement. “We working a plan to release walkers and those who are picked up by parents.”

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Associated Press writer Nick Perry contributed to this report in Meredith, New Hampshire.