Auburn police Officer Ronald Tarentino Jr. was making a traffic stop around 12:30 a.m. Sunday when an occupant of the car shot him and fled, police Chief Andrew Sluckis said.
Tarentino, 42, was taken to UMass Memorial Medical Center in Worcester, where he was pronounced dead, Sluckis said.
The shooting prompted a joint investigation involving several police departments, the Massachusetts State Police, the Worcester County district attorney's office and the FBI.
The search for Jorge Zambrano, 35, a suspect with an "extensive criminal history," led authorities to a duplex apartment in nearby Oxford, said Col. Richard McKeon, superintendent of the Massachusetts State Police.
Behind the residence they found a vehicle believed to be the one Tarentino stopped, McKeon said.
A state police tactical team entered one side of the duplex. The team cleared the apartment without finding the suspect, McKeon said.
Members of the tactical team found an opening in the cellar wall that led to the other side of the duplex, McKeon said. Investigators made their way through the cellar and the first floor, climbing to a bedroom on the second floor, he said.
As they entered, a closet door "burst open," and the suspect emerged firing at the officers, McKeon said.
"He was lying in ambush, waiting for them," Early said.
An exchange of gunfire ensued in which the trooper and Zambrano were shot. The trooper and Zambrano were brought to UMass Memorial Medical Center, Early said.
Zambrano was pronounced dead at the hospital, Early said. The trooper, an 18-year veteran and former U.S. Navy SEAL who has not been named, was injured in the shoulder, state police said. He had surgery Sunday night, the agency said.
While officers from various agencies moved through Auburn and neighboring towns to follow leads, others lined the streets to pay tribute to the slain officer as a motorcade brought his body from the medical examiner's office to a funeral home.
"We are devastated for his family. The residents of Auburn have lost a dedicated and brave public servant," Sluckis said.
Investigators don't know the motive for Tarentino's killing, according to Paul Jarvey, a spokesman for the Worcester County district attorney's office.
"We owe his family a thorough accounting of everything that happened," Sluckis said.
Tarentino, who joined the Auburn Police Department two years ago, is survived by his wife and three children.
A liaison officer from the Auburn police has been assigned to the Tarentino family until the officer is buried, Sluckis said.
"There was not a member of this department that didn't embrace him and like him. He got along with everybody, was somebody who was always smiling," Sluckis said.
"He was an outstanding guy, and we're going to miss him quite a lot."
Nineteen law enforcement officers have died in firearm-related incidents in the United States so far this year, according to the National Law Enforcement Officers Fund. Presumably, that number includes the Massachusetts death, as the fund's website said its total was through May 22.