PORTLAND – Asserting that a fatal shooting in Standish could be a case of self-defense, investigators plan to let a grand jury decide if anyone should be charged in connection with the killing.

State police say Matthew Perry, 29, shot Christopher Denis, 23, after he showed up Monday night at Perry’s mobile home off Route 114 after learning that his ex-girlfriend, Susan McNerney, and her teenage son were there.

The Attorney General’s Office, which reviewed preliminary evidence in the death, said letting the grand jury decide whether a homicide is justifiable is a procedure that has been used in past cases.

“Is there evidence here that raises the issue of self-defense or defense of premises? Yes, there is,” said Deputy Attorney General William Stokes, head of the criminal division.

Police questioned Perry after the shooting and then released him Tuesday morning, but the relationship between the two men remains under investigation. At least one of the two other people in the house witnessed the shooting, said Stephen McCausland, spokesman for the Department of Public Safety.

McCausland said detectives are looking at past arrests as they investigate Denis’ death. He added that he was unaware of any court orders that prevented contact between McNerney and Denis. The two had broken up in the past three weeks, police have said.

Both men have had dealings with police, though Perry’s contact primarily involved minor civil infractions.

The Cumberland County grand jury likely will hear the case next month.