FOX LAKE, Ill. — As a small northern Illinois community mourned a popular veteran police officer who was fatally shot while on duty, authorities scoured the area overnight in search of three men wanted in his slaying.

Helicopters were to aid about 100 officers early Wednesday as the manhunt in and around the village of Fox Lake for the suspects the fatal shooting of Lt. Charles Joseph Gliniewicz stretched toward a second day, a sheriff’s office spokesman said.

Authorities from across the state and region poured into the village about 60 miles north of Chicago after the shooting Tuesday morning, some wearing tactical gear and toting high-powered rifles.

Federal agencies, SWAT teams and 48 police dogs assisted in the search for the suspects, Lake County Sheriff’s Office spokesman Sgt. Christopher Covelli said.

Officers could be seen taking up positions on rooftops and along railroad tracks, scanning the terrain with rifle scopes and binoculars. Others leaned out of helicopters with weapons at the ready. Residents were urged to stay indoors, and schools were put on lockdown.

Authorities said Gliniewicz radioed in Tuesday morning to tell dispatchers he was chasing three men on foot in Fox Lake. Communication with him was lost soon after.