Police said Adaisha Miller was dancing with the officer when she hugged him from behind. His gun, which was in a waist holster, went off, and the bullet punctured Miller's lung and hit her heart. She died at a hospital.
Detroit Police Chief Ralph Godbee described Miller's death during a Monday news conference as a tragic, "unfathomable" accident.
"Somehow, in the course of dancing with the individual to his rear and touching his waist, his Detroit Police Department-issued weapon discharged, striking Ms. Miller," Godbee said. "There is absolutely no indication that the officer placed his hand on his weapon at all."
Godbee implied contact from Miller appeared to have caused the gun to go off, but he stopped short of saying she pulled the trigger on the .40-caliber handgun.
The officer, 38-year-old Isaac Parrish, has been placed on desk duty while the case is investigated. There was no answer at his home early Monday afternoon. Godbee said Parrish is "very remorseful."
Miller's mother, Yolanda McNair told The Associated Press her daughter would have turned 25 on Monday and she was celebrating the weekend before her birthday. Miller was invited to the Saturday night fish fry at Parrish's home by friends who knew him, her mother said.
"Why do you need a gun at your own house? Why do you need a gun at your own party?" McNair asked. She said she would be watching the police investigation closely.
Godbee said the officer's gun had a safety mechanism built into the trigger.
The Smith & Wesson M&P primarily was designed for police and military use. It does not have a safety switch, but the trigger has to be pulled back completely for the gun to fire, certified firearms instructor Rick Ector said.
Ector said that if properly holstered, the gun cannot be fire accidentally.
But Godbee said Parrish's waist holster was made of a soft material, and it would be possible for the trigger to be pulled while the gun was in it. He said the barrel direction typically would have been pointing down while holstered.
The gun's angle also is at question, according to a former Michigan State Police firearms examiner.
"What's going to be very important here is the angle of the entry of the wound to the victim (and) if there is in fact any gunpowder residue," Balash said. "I'm having a great deal of difficulty understanding how a weapon that's pointed at the ground can be turned literally 100 degrees minimum to be in an upward position to strike someone."
There still is "no justification" for her daughter's death, said McNair, 44.
"She didn't do anything wrong," McNair said. "She told us she was going to a party."
The Wayne County medical examiner's office was to perform an autopsy.
Associated Press writer Mike Householder contributed to this report.