GROTON -- Police are investigating how a 4-year-old boy drowned in a swimming pool Monday evening.

The boy was unresponsive after being pulled from a pool, according to police. Emergency crews responded to a 911 call for an unresponsive child who had been pulled from a pool about 7:20 p.m.

Adults who were there immediately began CPR on the boy, and off-duty EMS and firefighters took over seconds later, police said. Police officers and firefighters from Groton and Townsend, who quickly arrived at the scene, also tried to resuscitate the boy, who has not been identified, police said.

The boy was transported to the Nashoba Valley Medical Center in Ayer, where he was pronounced dead.

Groton police as well as state police assigned to the office of Middlesex District Attorney Gerry Leone are investigating the incident.

Police would not immediately release the exact location of the incident.