(Latest updates: Boyfriend of Md. teacher held without bond after body was found in shallow grave)

On Monday, Tyler Tessier soulfully pleaded for the safe return of his then-missing pregnant girlfriend, Laura Wallen, and held hands at a news conference with Wallen’s mother.

But by then, Wallen already had been killed by Tessier, according to police, who charged him Wednesday in the death of the 31-year-old high school teacher, whose body was found in a shallow grave in Damascus.

Tessier also was believed to be the father of Wallen’s unborn child, police said. She was four months pregnant. Wallen, of Olney, was a social studies teacher at Wilde Lake High School in Howard County, where she taught since 2014.

Since she was reported missing early last week, the case has received widespread attention. Wallen was excited about the school year starting and didn’t seem at all like a person who would take off on her own, family and friends said.

Laura Elizabeth Wallen (Montgomery County Police Department )

Behind the scenes, detectives were in constant contact with the boyfriend, Tessier, who gave inconsistent stories about his interactions with Wallen before she’d gone missing, police said.

When police held the news conference to ask the public for help finding Wallen, investigators made “a calculated decision” to allow Tessier to attend, Montgomery Police Chief Tom Manger said Wednesday night.

Tessier sat next to Wallen’s mother and father. Detectives were curious what he would say and how he would say it, according to Manger, who said the family was aware of that decision.

When Tessier spoke in front of a bank of cameras, he pleaded for Wallen to contact her family and asked for her safe return, saying, “Laura, if you’re listening, it doesn’t matter what’s happened. It doesn’t matter what type of trouble. There’s nothing we can’t fix together.”

Manger said there were no signs on Wallen’s remains to suggest how she was killed.

During the investigation, police learned Tessier was spending a lot of time in an area of Damascus and came to believe they should search there for Wallen’s body, police said. Police searched the area on Wednesday with cadaver dogs and found a freshly dug area, according to police, and in plain view saw a purple piece of fabric.

Police said they believe Wallen was killed Sept. 3, a day after a surveillance video at a grocery store showed her and Tessier together.

Tyler Tessier (Montgomery County Police)

Wallen’s sister received several odd texts from Wallen’s phone on Sept. 4, stating that the child she was carrying was not Tessier’s and suggesting it might be that of a previous boyfriend, Manger said.

Manger said that police determined those texts had been sent by Tessier.

“I am like 95 percent sure Tyler is not the father,” one of the texts said, according to police.

“I’m probably going to lose my job over this,” said another.

And yet another text stated: "Tyler is never going to forgive me. If he tries to call you, please tell him he's a great guy because I know I really hurt his feelings."

On Sept. 7, police found Wallen’s car unoccupied and parked in an apartment complex in the 10600 block of Gramercy Place in Columbia, a five-minute drive from the school. Police said Tessier brought the car there after removing the front license plate.

When Tessier spoke Monday at the news conference, in his appeal for Wallen, his voice occasionally broke as he said: “We haven’t slept, we haven’t eaten, we’re just looking, we’re praying that you’re safe. I’m asking you to just let us know that you’re safe. If somebody has her, please understand that you’ve taken away a huge person in so many people’s lives.”

Wallen’s body was found near Prices Distillery Road in Damascus on private property, police said. The property owner has “no involvement” in the investigation, Manger said.

Wallen’s father, Mark Wallen, had said he became certain something was awry when he called Wilde Lake High School on Tuesday morning to check if his daughter had arrived and was told she had not. He then asked if she had arranged for a substitute teacher and was told no — a lapse he said he was sure she would not have made had she known she would be absent.

He knew she had been preparing her classroom for two weeks and had a "joy in her heart" for her job, Mark Wallen said.

Tessier, at the news conference asking for public help in locating Wallen, said she was "super excited" for the start of the school year and called her disappearance a "complete shock." He said the two had gone to doctor appointments together and that she was happy about the pregnancy.

Police said Laura Wallen had sent her sister a series of texts Sept. 2 saying “Tyler has me on an adventure in the country. . . . don’t know why I’m here but it’s for something.”

Her sister asked, “Really where are you?”

Laura Wallen replied, according to police, “I’m waiting in a field.”

Her sister told her to take a picture. Police said the picture Wallen sent was of a large field with a tree line and that the photograph appears to be the same field where the clandestine grave was discovered.