The 31-year-old high school teacher had been reported missing September 5 by her family after she didn't show up for the first day of class at her Columbia, Md. school.
Police say that Tessier drove Wallen's auto to the apartment complex in Columbia, where the vehicle was later located.
Police said Tessier allegedly texted Wallen's sister from her phone saying: "the baby isn't Tyler's".
Police officials said that Wallen texted her sister that day, saying that Tessier was taking her on an "adventure" in the country. "Laura, if you're listening, it doesn't matter what's happened", he reportedly said.
"I pray that she's safe and that she comes back", Tessier told the gathered reporters. But the acquaintance declined to assist, Manger said. Tessier later admitted to moving the front tag off the vehicle, as well as throwing away Wallen's driver's license and phone. The two were seen together in surveillance footage at a grocery store near Olney's home on Saturday, Sept. 2. Police think those texts were actually sent by Tessier.
Tessier is being charged with First Degree Murder after police found Wallen's body buried in a wooded area in Damascus.
"If somebody has her, please understand that you've taken away a huge person in so many people's life", Tessier said Monday.
But police had a feeling that Tessier's story during the news conference wasn't the truth.
Pregnant Maryland teacher found dead in shallow grave, boyfriend charged with murder
Bond review today at 1:00 pm in Rockville District Court for Tyler Tessier, charged with first-degree murder of Laura Wallen. "We had a real sense of dread that this was the way it was going to end up".
On Monday, Tessier went on television with Wallen's parents to ask for his girlfriend's safe return.
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.
Manger said Tessier's appearance at the news conference was a "calculated decision" made by police and Wallen's family. The victim's family gave their approval.
The cause of death is unknown, Manger said.
Manger said it will be up to prosecutors whether to charge Tessier with the death of Wallen's unborn child. That woman has been interviewed by police, but Manger was unable to elaborate on what was discovered.
After the high school teacher went missing, the case had received widespread attention.
A mobile crisis team has been established at Wilde Lake High School to support the community following Wallen's murder.
The school's "Gathering of Hope" event scheduled for Wednesday night has reportedly been canceled.More news: Olympic host cities Paris and Los Angeles
More news: Target ups holiday hiring by 40%
More news: Real Madrid coach Zinedine Zidane says getting Ronaldo back is a relief