For a week, as police in suburban Maryland searched for a missing, pregnant high school teacher, they grew convinced her boyfriend knew more than he was saying. Then they found the body of the teacher, shot once in the back of the head and buried in a shallow grave several hundred yards from where the boyfriend lived.

The boyfriend, Tyler Tessier, offered an explanation: Several men had kidnapped him and his girlfriend, Laura Wallen, killing her but sparing him.

Then he offered another, according to police: He and Wallen had argued, and as she lunged at him, she ran into a wooden post and collapsed, appearing to be dead. He then buried her in a field. In trying to explain why she was found shot, Tessier allegedly told detectives he had worried he might have buried her alive and fired one round at her to make sure she was not suffering.

The explanations offered by Tessier — as spelled out in court documents recently filed by Montgomery County prosecutors — describe a homicide suspect who had painted himself into a corner. His version of why he shot Wallen, according to filings first reported by WTOP, have him putting the weapon in his own hand.

Tessier is scheduled to be tried in Wallen’s slaying starting Sept. 4. Authorities say he killed Wallen after she tried to contact a different woman he was dating. Tessier did so by luring Wallen to the field, where he shot her and buried her, according to prosecutors. His attorney has said he is innocent.

Wallen, who had lived in Olney, went missing the weekend of Sept. 2 after being last seen with Tessier. She became the object of a massive, week-long search by police, including a news conference at which Tessier pleaded for her return, addressing her directly and sitting beside her parents.

“Laura, if you’re listening,” Tessier said in front of a bank of cameras, “it doesn’t matter what’s happened. It doesn’t matter what type of trouble. There’s nothing we can’t fix together.”

Wallen, 31, was a social studies teacher at Wilde Lake High School in Howard County, where she failed to show up for the first day of 2017 classes after several weeks of preparing her classroom.

On Sept. 13, two days after Tessier’s public appeal, police found Wallen’s body in the Damascus area of Montgomery County, arrested Tessier and interrogated him.

How they questioned him, what they asked him and whether they improperly kept Tessier from consulting a lawyer are at the root of the recently filed court documents.

In a March 9 filing, Tessier’s attorney, Allen Wolf, argued that Tessier’s statements while he was in the interrogation room should not be allowed as trial evidence.

Wolf said in the filings that investigators zeroed in on his client early.

“Tyler Tessier became a target of police investigation almost immediately after his pregnant girlfriend, Laura Wallen, failed to report to her job on the first day of school,” Wolf wrote. “Over the following week, police interviewed Mr. Tessier on multiple occasions, both at the police station and at a Dunkin Donuts.”

Each time, Wolf wrote, Tessier was allowed to leave.

After Wallen’s body was found, Tessier was taken to police headquarters and handcuffed to a table in an interrogation room, Wolf wrote, and eventually two detectives entered.

One detective asked: “Do you want to try to explain why her body wound up” in the field?

“Can I talk to my lawyer please?” Tessier said, according to Wolf’s filing. “Can I talk to my lawyer?”

Wolf said the detectives already knew that Tessier was represented by an attorney, yet proceeded to talk him out of trying to speak with the lawyer.

“You can talk to your lawyer whenever you want,” a detective said. “But here’s what I have to tell you. This, this is it. You’re not going to see the two of us again. . . . You can tell us whatever you want to tell us.”

In response to Wolf’s filing, prosecutors from the Montgomery State’s Attorney’s Office wrote that Tessier may have consulted with an attorney before Wallen’s body was found, but he had not retained an attorney. And although he asked to speak to an attorney, prosecutors argued, Tessier on his own started talking to the detectives.

“I am more than willing to tell you everything you want to know,” he said, according to the prosecutors’ filing.

“The defendant consistently claimed he had not hurt Ms. Wallen in any way,” prosecutors Donna Fenton and Mary Herdman wrote as they explained Tessier’s interactions with police while the search for Wallen continued.

But after the news conference, Tessier met with detectives and appeared to change his story. “The defendant admitted to helping Ms. Wallen ‘disappear,’ ” according to prosecutors’ accounts.

He told detectives he moved her car around and got rid of the license plate and her telephone, according to the prosecutors’ filings.

Tessier claimed in that session, prosecutors wrote, that the teacher wanted to disappear because she was carrying a baby that was not his and feared she could lose her job over that alleged relationship. Tessier said, as he gave that version of events, that he had not hurt her, prosecutors wrote.

Those explanations from Tessier changed again, detectives said, after Wallen’s body was found.

His final story, according to prosecutors’ court filing, was about her collapse after hitting a porch pole — and his shooting her after he worried that he had buried her alive and in agony.

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