On Thursday evening, police confirmed beloved teacher Laura Wallen was shot in the back of the head. They aren't saying that is the cause of death, but they are still looking for the weapon.

Prosecutors say they believe the motive behind the murder of a Maryland high school teacher was a love triangle.

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Tyler Lewis Tessier, 32, accused of killing his girlfriend, Laura Wallen, 31, admitted to police that he was engaged to another woman, court documents show. Both women were aware of each other, however believed they were dating him exclusively.

The investigation reveals that at the end of August, Wallen sent the following text message to Tessier's fiance:

"It's important that some things are cleared up and I would imagine that if you were in my position, you'd want some answers as well. By no means is this an attempt at confrontation, just looking for an explanation...woman to woman."

Prosecutors believe the love triangle between Wallen, Tessier and his fiance was the probable motive behind the killing. Wallen's attempt to get in touch with the fiance may have also played a role in her murder.

On Wednesday evening police announced Tessier's arrest. This came just days after a press conference where Tessier tearfully pleaded for the return of his pregnant girlfriend. He sat next to Waller's parents, holding the hand of her mother.

Police say it was a 'calculated decision' by detectives to allow him to participate in the press conference to see what Tessier would say. Former Prince George's County State's Attorney Glenn Ivey says sometimes police use strange tactics to get people to talk.

"I assume what they had in mind was to get him talking and to have him speak freely without realizing he was a suspect," said Ivey.

Wallen's parents knew from the beginning of the investigation police suspected Tessier, playing along with police during the press conference.

"He is a monster and a liar. It was the hardest thing my wife could do to sit next to him and hold his hand," said Mark Wallen, Laura's father, after Tessier's first court appearance Thursday.

Court documents revealed Wallen's best friends were also led to believe that her baby had been fathered by another man. According to court documents, Tessier sent messages to them pretending to be Wallen, stating that she was worried that somebody else was the father.

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Furthermore, one of Wallen's best friends received a series of text messages from her on September 2, court documents show. Wallen sent a text to her best friend saying that Tessier had taken her to a field and she didn't know why. She then sent a picture of the large field with a tree line. Authorities think the picture was of the same field where her body was later found.

Tessier went back to that field at least ten times since Wallen went missing, according to the investigation.

Wallen last contacted her family on September 3, officials said. Her body was found on Wednesday in a shallow grave in the Damascus area.

Tessier, was arrested for Wallen's murder on Wednesday. He is being held without bail.

Read through the statement of charges here:

Tyler Tessier Statement of Charges by wusa9 on Scribd