Transcript for New details emerge in alleged murder of pregnant Maryland teacher by boyfriend
Haven't slept, we haven't eaten, we're just looking. We're praying that your safe. Reporter: A boyfriend's tearful plea for the return of his girlfriend, four months pregnant with his child. I don't know where she is. That's all. I don't know. Reporter: One week earlier, 31-year-old Laura Wallen, a beloved high school social studies teacher, had gone missing. Laura, if you're listening, doesn't matter what type of trouble. There's nothing we can't fix together. Reporter: What tissier didn't know is Maryland police suspected Laura was already dead and he might be the killer. Police staged the press conference on Monday hoping Tessier might give away revealing details. It was a very calculated decision to ask him to be a part of that press event. Reporter: In on the setup, Laura's own family. Her mother even holding Tessier's hand. He is a monster. He is a liar. It was absolutely the hardest thing that my wife could do, to sit next to him and hold his hand. Reporter: Two days later, police would find Laura's body buried in a shallow grave in a field belonging to a friend of Tessier's. Autopsy was done on the victim's body and it was determined that she had, in fact, been shot in the back of the head. Reporter: And just hours later -- At 5:30 P.M. This afternoon, Tyler Tessier, Mrs. Wallen's boyfriend, was arrested for her murder. Since Ms. Wallen was reported missing by her family nine days ago, we've determined that Mr. Tessier was the last person known to have been with the victim. Reporter: According to the police investigation, Laura was last seen alive on September 2nd at a grocery store near her home with Tessier. Texts she sent helped police piece together other crucial details about just what happened on that day. Tyler has me on an adventure in the country, don't know why I'm here but it's for something, I'm waiting in a field. Laura's sister texted her to take a picture. Two days later police say Laura's sister got suspicious text messages from Laura's home. I'm like 95% sure Tyler is not the father, I'm going to try and get a hold of atoine. Her sister says Laura hadn't spoken with her ex-boyfriend Antoine in two years. And that his name was spelled incorrectly. Certainly Laura knew how to spell the name. The text spelled the name wrong. Jimmy: Stranger still, Laura's family says from the beginning of her pregnancy she said Tessier think it was the father. Police now think it was he who sent those texts, not Laura. It's pretty unlikely a woman is going to send out text messages describing the fact that she really doesn't have any idea who the baby's father is. And that's going to resonate with females on the jury. The fact that he was trying to text her to give the family some indication that she was okay, certainly was an indication to us that he was trying to take us off the track of this investigation. Reporter: According to T ech Tessier's arrest warrant, the same morning of those texts Laura's driver's license turned up near the dumpster of an apartment complex parking lot. Three days later the property manager contacted the police after recognizing the photo from a missing person post on Facebook. Police then found Laura's 2011 Ford escape in that same parking lot. This footage captured by CBS station W Jay-Z. But the front license plate was missing. Somebody wanted her trail to go cold. Someone did not want her car to be identified or found. Someon wanted to hide evidence. Reporter: Throughout the week, Tessier was questioned several times, always sticking to his story he last saw her alive on September 4th. Police say he provided conflicting statements about other details and during the last interview, police say he made several critical admissions. One was the fact that he, in fact, took the front license plate off of her car. He admitted that he threw her driver's license and her cell phone in a dumpster. He gave us an excuse that he was trying to help her because he said that she was going up to confront an ex-boyfriend, which was a lie. Reporter: With suspicion growing, the police stage that press conference. There's so many people, so many people that miss you. You put the suspect in front of a camera because he's going to talk. And he may inadvertently say something or present something in a way that you know is not true. Reporter: But it was Laura's last text to her sister that helped lead police to this field. Tessier also told police he visited multiple times since Laura went missing. Our search team did search the perimeter of those entire woods and at some point noticed tire tracks that went into an area of the woods and follow -- and that's what led them to the area where we recovered her body. Reporter: Police say they still don't know Tessier's motive but there may have been another woman in his life that Laura may have known about. Days before she went missing, she texted the alleged other woman saying, in part, I would imagine if you were in my position you'd want some answers as well. Just looking for an explanation woman to woman. The story of a possible love triangle gone tragically wrong. Strikingly similar to Laci Peterson's murder in 2002. The 27-year-old, 7 1/2 months pregnant, went missing on Christmas eve and made national news headlines. A California man is anxiously waiting to hear word on the mysterious Christmas eve disappearance of his pregnant wife. Reporter: Her husband Scott quickly became a person of interest, giving this interview to ABC's Diane sawyer. Are you afraid police will arrest you? No. I know there is -- there's no basis. I mean, I had nothing to do with her disappearance. So there's no possible evidence or anything like that. Reporter: Soon Scott's girlfriend, amber Frey, comes forward. Scott told me he was not married. We did Vroman tick relationship. Reporter: Scott Peterson was given the death penalty. And is currently appealing his sentence. Tyler Tessier is now also behind bars, arrested Wednesday. He's charged with Laura's murder, destruction of evidence, and providing false information. His bond has been denied. And he's not yet entered a plea. In this jurisdiction, it will all boil down to, was the baby viable? At the time of the homicide? If the baby was viable, this is a double murder. If not, just one premeditated malice murder. Reporter: Tessier's public defender did not return requests for comment. As for Laura's family, they're now left mourning the loss of two lives. God did not give us the miracle that I asked for. But he did give us a miracle nonetheless. We have our daughter back. Reporter: For "Nightline," I'm linsey Davis in New York.
This transcript has been automatically generated and may not be 100% accurate.