Patrick Frazee, the Colorado man accused of killing his fiancée, was charged on Monday with two counts of first-degree murder by prosecutors who posited differing theories about how she died.

Prosecutors say Frazee either killed Kelsey Berreth, the mother of his 1-year-old daughter, or she died during a robbery he commissioned.

On Monday, Frazee was charged with two counts of first-degree murder and three counts of solicitation to commit murder in the first degree.

Berreth's body has not been found, but investigators don't believe she is alive. She was last seen on Thanksgiving shopping with her daughter, Kaylee, at a Safeway grocery store in her hometown of Woodland Park.

Booking photo of Patrick Frazee released by Woodland Park Police. Woodland Park Police

A charge sheet released Monday suggests that Frazee may have ordered a robbery between Sept. 1 and Dec. 1, which resulted in Berreth's murder. Under Colorado law, if a person dies during the commission of a crime, that is equivalent to murder, 4th Judicial District Attorney Dan May said during a press conference on Monday.

“Obviously, there’s only one murder in this case. Under Colorado law, we file separate counts under different theories of first-degree murder,” May said.

May said the law requires the prosecution to differentiate between its theories of murder. One theory from the prosecution says Berreth was killed during the commissioning of a crime. The other says Frazee premeditated the killing of Berreth.

The public defender's office did not immediately return a request for comment made by NBC News. May's office also did not return a request for comment.

Berreth was first reported as missing by her mother, Cheryl, on Dec. 2.

"Never has she gone anywhere without saying anything," Cheryl said. "It's just not like her."

Officials said Berreth's phone pinged to a location near Gooding, Idaho, about 800 miles away, around 5:15 p.m. on Nov. 25 — the last day Frazee said he had contact with her.

Berreth's employee, Doss Aviation, also received a text message from her phone on Nov. 25, police said. She told the company, where she works as a flight instructor, that she needed time off.

Because the case is still being investigated, and it is possible other people could be charged in Berreth's disappearance, all affidavits in the case are still sealed. Because of this, it is unclear what evidence the prosecution has against Frazee.

However, May said the judge overseeing Frazee's case has seen the arrest affidavit and believes there is probable cause to charge Frazee with first-degree murder.

May declined to specify if Frazee allegedly conspired with one or more people and the charges do not specify who those people might be.

A hearing on the testing of evidence will take place on Jan. 4. A preliminary hearing where evidence and testimony will be presented is scheduled for Jan. 29.