KANSAS CITY, KAN. ? A Leavenworth man already serving 35 years on kidnapping and firearms convictions was convicted Thursday in federal court in Kansas City of killing three people, including a man he once kidnapped, and conspiring to kill a fourth.

After a six-week federal trial, jurors found Demetrius Hargrove, 31, guilty of three capital murder counts and one count of conspiracy to kill a federal witness. Those convictions could result in the death penalty.

Two of the counts stemmed from the February 1998 killing of Elmer Berg Jr., 23, and his sister, Misty Castor, 22, both of Leavenworth, during a drug trafficking crime.

The third capital murder count was for the July 1998 death of Tyrone Richards, 27. Prosecutors said Richards was killed to prevent him from testifying in a federal trial for Christopher Trotter, who also had been accused of kidnapping him.

Several months before Richards was killed, he was taken at gunpoint from Leavenworth into Missouri and then returned to Kansas, where he was rescued when police pulled over the kidnappers’ car. Prosecutors said Hargrove and Trotter escaped.

Both Trotter and Hargrove were charged with kidnapping Richards to keep him from talking to authorities about other criminal activities. Trotter later was tried and convicted, but was granted a new trial.

The government eventually dropped the kidnapping charge against Trotter because he cooperated and testified in the Berg and Castor murder case. Trotter was sentenced in July 2003 to life in prison for killing a pregnant woman and the father of her three children in an unrelated botched robbery at their home.

Hargrove’s conviction for conspiracy to kill a federal witness stemmed from jailhouse phone calls he made to arrange the killing of Shedrick Kimbrel to prevent him from testifying in Richards’ kidnapping, prosecutors said.