Three police forces and the US Coast Guard on Saturday night chased down a Medford-area man who faces the possibility of federal charges after allegedly pointing a hand-held laser beam at a helicopter.

"What goes through one's mind? . . . You never know what's on the other end of a laser," said Sergeant Robert Bousquet of the State Police.

State Police pilots were on a routine security mission, guarding a liquid natural gas tanker near shore at about 9:15 p.m. Saturday night, Bousquet said. They noticed a laser light touching their helicopter.

Using sophisticated equipment on their helicopter, including cameras, they pinpointed the origin of the light in the Medford-Somerville area, Bousquet said.

The search was on - involving authorities from the Coast Guard, the State Police, the Medford Police Department, and the Somerville Police Department. About 90 minutes later, police found an adult male.

Bousquet would not say exactly how police located him, nor would he identify the man because charges have not been filed, though "illuminating an aircraft" is a federal offense. He said the investigation was turned over to federal officials.

The light never interfered with the pilots' vision, nor is there any link to a terrorist threat, Bousquet said.

Even if it was an ill-advised prank, Bousquet said, the possibilities for criminal intent are broad when a pilot is faced with a laser beam.

© Copyright 2007 Globe Newspaper Company.