A teen fatally injured his ex-girlfriend inside his Long Island home Wednesday before turning the weapon on himself — and surviving, according to authorities.

The unidentified 17-year-old caused the death of his ex-girlfriend, Emily Finn, 18, inside his home in Nesconset around 11:10 a.m. on Wednesday, the Suffolk County Police Department said in a statement.

The teen then seriously injured himself, police said.

His parents called 911 after loud bangs were heard, according to authorities.

Finn, who had just graduated from Sayville High School in June, was pronounced deceased at the scene, police said.

She had gone to her former boyfriend’s home to return some of his belongings following their recent breakup, Suffolk County Det. Lt. Kevin Beyrer, commanding officer of the department’s major crimes bureau, confirmed to Greater Long Island.

The 17-year-old was taken to Stony Brook University Hospital in critical but stable condition, authorities added.

He is expected to face second-degree homicide charges and will be arraigned at a later date, police said.

There was no prior “domestic history” between the teens or emergency calls at the Nesconset home involving Finn or the individual involved, police said at the scene, according to the outlet.

A neighbor told The Post that the teen’s family was a “typical” household and involved with the community.

“They’re just a great, typical community-involved family. They’re a normal family, and now multiple families will be devastated by this,” the resident, who wished not to be identified, said.

Another neighbor, who did not know the family personally, was shocked to hear of the incident occurring so close to home and on the day before Thanksgiving.

“It’s terrible. It’s sad, and you never think it will hit so close to home. It’s shocking,” the neighbor said.

The investigation remains ongoing, and a motive has not yet been determined.