MARYLAND -- He's accused of leading his pregnant girlfriend out to the middle of a field and shooting her in the back of the head.

On Friday, the Montgomery County prosecutor announced he's unable to bring a second charge of murder against Tyler Tessier in the death of his unborn child.

“Based on medical testimony,” said State’s Attorney John McCarthy, “The child was not viable independently of (his mother, Laura) Wallen.”

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Homicide laws in Maryland and many other states only cover viable fetuses. That's based on the idea that the courts cannot protect a fetus that a woman has a constitutional right to abort.

Investigators say Laura Wallen's last texts were from an empty field in Damascus.

"Tyler has me on an adventure in the country...Don't know why I'm here, but it's for something," she text to her sister.

"Really, where are you?," her sister texted back.

"I'm waiting in a field," she replied.

Investigators say Tessier shot her in the back of the head and buried her there. And then appeared at a news conference with her family, who were desperately searching for her.

“I don't know where she is,” he said. “That's all. I don't know where she is.”

Her students at Wilde Lake High School in Howard County held a vigil after learning she'd been murdered.

“How could you be possessed with so much hate to take a soul that meant so much to people?,” asked one teen as he cried softly.

Detectives say Tessier was in engaged to another woman and killed Wallen after she learned about it.

A grand jury just indicted Tessier in the murder of Wallen. But the prosecutor now says he cannot charge Tessier with also murdering the 14-week-old baby -- because Maryland law in murder cases requires a fetus to be viable outside the womb.

“Based on those medical findings and as a matter of law in Maryland,” said McCarthy, “the defendant is not chargeable with a second count of murder related to the child.”

The CEO of the American Congress of Obstetricians and Gynecologists told The Washington Post that a 14- week-old fetus has "zero" chance of surviving outside the womb. Dr. Hal Lawrence told the paper even a 23- week- old fetus rarely survives. Case law in Maryland suggests the standard in murder cases is 27 weeks.

If convicted of the single count of murdering Wallen, Tessier faces a maximum life sentence. Prosecutors are still trying to determine if murdering the unborn child is an aggravating circumstance serious enough that they should ask for life without the possibility of parole.

Tessier's lawyer said he's innocent and that he would never have hurt her.