ALAMEDA — The district attorney's office has decided not to file charges against an Alameda police K-9 officer whose dog died when he left it unattended while he was on duty.

"It is very tragic when an animal dies under these circumstances, but there is insufficient evidence to prove that the officer acted unreasonably based upon all the evidence presented in the case," Assistant District Attorney Charlotte Green said in a written statement on Monday.

An internal police investigation into the actions of the officer, who has not been named, remains pending.

The officer was attending a training exercise May 5 when he left the animal in his Sport Utility Vehicle for about three hours and 15 minutes, police said.

When the officer returned to the SUV, which was in a parking lot in the 2200 block of Lincoln Avenue adjacent to the police station, he found the 6 1/2-year Belgian Malinois in "distress," Alameda police Lt. Bill Scott said.

The dog was brought to the Bay Area Veterinary Emergency Clinic in San Leandro, where it died.

The officer remains on duty.

Along with not naming the officer, investigators declined to say how long he has worked for the department.