Berreth was last seen in public on November 22, when surveillance camera video caught her entering a grocery in Woodland Park, about 90 miles southwest of Denver.
Police have said Frazee was the last person to see Berreth alive.
Frazee will make his first court appearance by video on Friday afternoon, Colorado judicial spokesman Rob McCallum said.
She said the couple had weathered some financial struggles.
"As you can tell from the arrest, sadly, we do not believe that Kelsey is still alive", De Young said. Frazee's attorney, Jeremy Loew, said Frazee was cooperating with investigators and had submitted to a DNA test and a search of his cell phone.
"We finally received enough information to implicate Patrick in his involvement in Kelsey's death", De Young said.
"I understand why they can't provide them", says JoDee Garretson, 50, a first cousin to Berreth's mother, Cheryl, who first reported her daughter missing on December 2, 10 days after she disappeared.
Last weekend, authorities searched the property of Berreth's fiancé in Florissant, but they did not find any signs of the missing woman. But he stated that the police were "considering every possibility", though he stopped short of naming Frazee as a suspect.
Frazee was arrested at his home in Florissant just after daybreak on Friday and booked into the Teller County Jail.
Frazee, 32, was arrested at his home in the alpine town of Florissant on suspicion of murder and solicitation of murder, said Miles de Young, chief of police in neighboring Woodland Park, where Berreth lived.
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Kelsey Berreth was last seen near her home in central Colorado.
Berreth's phone was tracked to a location near Gooding, Idaho.
Police also said Doss Aviation had accounted for all their planes and there was no reason to believe she used someone else's plane for a flight.
The criminal affidavit in the case has been sealed until investigators can follow up on leads and feel confident the release of documents won't hurt the investigation, May said. He said more arrests could happen in the investigation.
De Young said to expect law enforcement in the area throughout the coming days and weeks.
Anyone with information is asked to contact the Woodland Park Police Department at 719-687-9262 or the Colorado Bureau of Investigation at 719-647-5999.
Berreth's phone did end up in Idaho, De Young confirmed Friday, adding that police are still working to recover it.
Earlier in the investigation, police said Berreth's cellphone had pinged a tower in Idaho, roughly 600 miles away.
Berreth's mother told CBS News' Nikki Battiste last week the couple was excited to get married, but hadn't set a wedding date.
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