LAWRENCE (CBS) – The trial of a Lawrence teenager charged with beheading a high school classmate began Monday. The Essex District Attorney says Mathew Borges stabbed and beheaded 16-year-old Lee Viloria-Paulino in November 2016.

Borges was 15 at the time. Both boys were sophomores at Lawrence High School.

Now, at age 18, Borges will be tried as an adult before a jury in Salem Superior Court.

Viloria-Paulino’s decapitated body was found on December 1, 2016, on the banks of the Merrimack River in Lawrence by a dog-walker. Borges said in a police report that they had gone to smoke marijuana together on November 18, then parted ways.

A jury of nine women and seven men listened to opening statements Monday morning.

The prosecution opened with a text message sent by Borges where he talked about killing someone before the murder.

Commonwealth Attorney James Gubitose added, “The defendant was tough on the outside but insecure and jealous on the inside.” Borges had allegedly grown insecure when he saw his girlfriend sitting with Viloria-Paulino at school and became enraged.

A notebook shared by Borges and his girlfriend will be used as evidence.

Defense attorney Edward Hayden told the court the details of the journal entries aren’t completely accurate therefore the notebook is unreliable.

“There is no murder weapon, there is no tools to dismember a body, there are no fingerprints, there is no blood, there is no DNA,” said Hayden.

Following open statements, 18-year-old Leilany Dejesus, an ex-girlfriend of Borges and another woman who Borges was talking to, 18-year-old Stephanie Soriano, took the witness testified.

Journal entries, text messages, Facebook messenger conversations, and audio messages were shared with the court.

Dejesus identified the notebook that she and Borges shared during their nine-month relationship. She confirmed he wrote the entry that said, “go chill with him at his crib alone drink and smoke and play it off and kill him call j.m. and scooby tell them to pull up and take everything bring duffle bags wear gloves clean up mess wear bags on shoes and wear clothes you don’t care about.”

Viloria-Paulino was described by Soriano as “a really good person at heart. He was the type of friend that you’d want to have around. He was very positive, very nonjudgement. Everybody would want him as a friend… Just a good soul. A sun, a very bright sunshine.”

Soriano read from a text conversation with her where Borges allegedly wrote, “I think of killing someone and I smirk. I like the sound of it. The idea of causing pain on someone who is getting in my way or who is causing me pain…It’s all I think about every day but I control myself. I see people I don’t like and it comes to mind.”

In an audio recording, Borges warns the girl that he could soon have “dead eyes.”

A family friend of the Viloria-Paulinos said, “If you use your mind, one person like that guy, only a little kid, don’t do something like that alone.”

The district attorney expects the trial will take about a month.