The Mastic man charged with stabbing a teenager to death last month may have mental illnesses that played a role in the attack, his attorney said Tuesday.

"That's something I will explore for a possible defense," attorney Phillip Murphy of Bay Shore said after his client, Michael Dennis, 19, pleaded not guilty to first-degree manslaughter.

Dennis is accused of killing Trevor Bender, 17, also of Mastic, on May 29. State Supreme Court Justice Mark Cohen ordered Dennis held without bail.

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Assistant District Attorney Kathleen Kearon said a car in which Bender was a passenger was stuck in traffic in front of Dennis' mother's house that evening. Dennis and Bender, who had an ongoing dispute, began arguing, egged on by Dennis' sister, Kearon said.

Dennis lunged toward Bender, who had no weapon, and stabbed him once in the chest, Kearon said. Dennis fled and was arrested the next day, giving a name, address and birth date that weren't his, she said.

Kearon said Dennis has a considerable criminal history, including violence toward his mother.

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After the Riverhead arraignment, Murphy said his client may have attention deficit disorder or bipolar disorder, either of which could have affected his ability to keep himself under control in the heat of the moment.

Dennis "didn't have a conscious objective of killing him," Murphy said. "He wanted to terminate the confrontation."