The crew of a State Police helicopter narrowly avoided a “catastrophic’’ midair collision with a large drone that came within 100 feet of the troopers before suddenly falling to the ground, where it could not be located, State Police said.

State Police helicopter designated Air 2 was on a routine training flight from the Holliston area to Newburyport and was above the Lawrence area when the crew spotted the drone around 11:10 a.m., State Police said.

“The pilot had to bank hard left to make sure he didn’t hit it,’’ said State Police Major Richard Prior at an afternoon press briefing in Lawrence.

Prior said the helicopter was traveling at about 120 miles per hour about 600 feet above the ground in the area of Interstate 495, River Street, and the Lawrence Municipal Airport when the drone was spotted. The drone, he said, was seconds away from colliding with the helicopter.

The crew in the helicopter initially thought it was a bird, he said. The drone was about two feet to three feet in length, according to Prior.

The pilot avoided a collision that could have been “catastrophic,’’ especially if the drone had flown through the windshield of the helicopter, said Prior.

The drone, he said, was in illegal airspace.

“It’s against the law to fly a drone in that vicinity,’’ he said.

“The drone got as close as approximately 100 feet in front of the helicopter before suddenly dropping to the ground,’’ State Police spokesman David Procopio wrote in an e-mail Wednesday.

Federal Aviation Administration rules limit flight ceilings for drones to 400 feet. The helicopter was within 5 miles of Lawrence Municipal Airport at the time, which means it was in a zone where drones are banned, according to State Police.

Danny McDonald of Globe staff contributed to this report. Rocheleau can be reached at matthew.rocheleau@globe.com.