Berkeley County Prosecutor Pamela Games-Neely declined to provide information about the alleged kidnapping.
If Deneen abides by the conditions outlined in the plea, he will be sentenced to 40 years in prison for the murder charge, the maximum sentence possible. He also will be sentenced to serve 40 years for the kidnapping charge. The sentences will run at the same time, Games-Neely said.
The plea was outlined before a judge in court Friday morning.
One of Deneen's attorneys, David Camilletti, said he discussed the plea with his client, but he declined to give a statement on Deneen's behalf.
Deneen is being held in Eastern Regional Jail without bail.
A judge is set to preside over Deneen's plea hearing in February. In the meantime, Deneen must give what Games-Neely called "a full recitation," which she said should include an admission of guilt and an explanation of how Deneen killed Frey.
Camilletti, though, described it another way, saying Deneen must admit facts that are sufficient enough for a judge to find the crime occurred.
Deneen must also provide a full and complete proffer, meaning he must tell investigators everything he knows about the two crimes, Games-Neely said.
Should Deneen fail to comply with the terms, Games-Neely said the plea could be thrown out by a judge.
Deneen's original March trial date for the murder charge remains open should the plea fall through, Games-Neely said.
In exchange for the plea, Washington County will not press charges related to the kidnapping case, Games-Neely said. Deputies with the Washington County Sheriff's Department, along with the West Virginia State Police, are investigating that case, Games-Neely said.
Deneen originally was set to appear before a jury for trial last month, but the case was rescheduled after new entomology and DNA evidence became known.
That evidence connected Deneen to the kidnapping, Games-Neely said, but brought up new questions about when and where Frey was killed.
"There have been holes in this case" that both sides recognized, Games-Neely said. "If the information we have up front is accurate, then it fits within a second-degree murder."
Police allege Deneen and Frey left Duke's Tavern in Hagerstown in the early morning of June 29, 2001.
Deneen, after first saying he did not know Frey, later told police he did, and drove her to a remote spot in Maryland to have sex, according to court records. He said that afterward, he dropped her off in Hagerstown and went to his girlfriend's home, records show.
Deneen allegedly asked his uncle to provide an alibi for him for the night Frey was killed, after saying he had killed a woman. Deneen also allegedly showed his uncle the knife he used, which police found in the rafters of an abandoned home in Clear Spring, Md.