STERLING, Colo. (AP) — Investigators have arrested a 19-year-old and his girlfriend on suspicion of murdering his grandparents in Sterling.

Brendan Lee Johnson and 18-year-old Cassandra Ann Reib could face charges of murder, forgery and robbery.

The couple is suspected in the deaths of Charles and Shirley Severance, both 70. The Severances are Johnson’s grandparents.

The body of Charles Severance was discovered in his home May 29. An autopsy showed no evidence of injury or trauma.

Police announced Tuesday that the remains of Shirley Severance were discovered near Sterling, with other remains found in Lorenzo, Nebraska.

Police did not immediately speculate on a motive. The two are scheduled to appear in court for advisement Wednesday.