A 21-year-old man from New York who allegedly killed his 17-year-old girlfriend -- and then posted photos of her body on Instagram -- might have done so out of jealousy after she reportedly kissed another man while at a concert Saturday night, officials said.

Oneida County District Attorney Scott McNamara said that the prosecuting office will likely treat the murder of Bianca Devins, 17, as a case of “extreme emotional disturbance” after an early investigation into the events that led to her killing suggest that the suspect, Brandon Clark, 21, didn’t plan it.

“Under the law, there's something called extreme emotional disturbance," McNamara told WKTV. "It doesn't negate the crime, but it mitigates it.”

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He went on to say that it would likely reduce the level of crime Clark would face, bringing his charge of second-degree murder down to “manslaughter in the first degree.”

McNamara told the local station that investigators believe Bianca “kissed somebody at the concert and that's what upset him.”

Bianca Devins was killed by a man she’d met recently on Instagram, who then posted photos of her corpse online, police said. (Instagram)

Officials said the pair attended a concert in New York City the night before the killing and it is believed that they got into an argument on the way home.

"I have a very hard time understanding how anybody can justify taking another young person's life because they kissed somebody else,” McNamara said.

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Bianca’s body was discovered by police Sunday outside a vehicle in Utica, along with Clark who was found seriously injured. He was hospitalized and is expected to survive. While It was not immediately clear what type of injuries he was suffering from, police said they were investigating the case as a murder and attempted suicide.

Utica Police said in a statement to Fox News: “With respect to their relationship, it was learned that the two had met on the social media platform Instagram approximately two months ago. They utilized this as a means of communication primarily, and their relationship progressed into a personally intimate one. They had spent time together, and were acquainted with each other’s families.”

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Investigators believe the man posted pictures of the girl’s corpse on Instagram, where at least one was widely circulated.

Clark is still currently facing a second-degree murder charge.

Fox News’ Frank Miles contributed to this report.