A former missionary from Oklahoma convicted of sexually abusing children at an orphanage in Kenya was sentenced Monday to 40 years in a federal prison.

U.S. District Judge David L. Russell handed down the sentence to Matthew Lane Durham, 21, who had faced up to 30 years on each of four counts of engaging in illicit sexual conduct in foreign places. He also ordered Durham, of Edmond, to pay restitution of $15,863.

Durham showed no emotion when the sentence was issued.

"These were heinous crimes committed on the most vulnerable victims. He was their worst nightmare come true," Russell said.

Durham asked the court for mercy prior to the judge's order.

"All I wanted was to follow God's plan for me," he told the judge.

Prosecutors alleged Durham targeted orphans while volunteering at the Upendo Children's Home in Nairobi between April and June 2014. Durham had served as a volunteer since 2012 at the orphanage, which specializes in caring for neglected children.

A 12-member jury convicted Durham in June on seven counts of engaging in illicit sexual conduct in foreign places, but Russell acquitted Durham on three of the charges in January.

The same jury cleared Durham of accusations he planned to abuse the children before leaving the United States. Defense attorney Stephen Jones has said Durham plans to appeal his convictions.

Orphanage officials and five of the children traveled from Kenya to testify at the trial. The children, who speak Swahili, testified through an interpreter only after Russell cleared the gallery and closed the courtroom to the public and media.

In a sentencing memorandum, federal prosecutors asked Russell to sentence Durham to 120 years in prison — the maximum punishment he faced. Prosecutors also asked that Durham be placed under supervision for the rest of his life in the event he is ever released from prison.

"The defendant's offenses were undoubtedly serious. He raped or sexually molested by force or threat four children ranging in ages from 5 years to 14 years — some multiple times — in a span of just 33 days," prosecutors wrote in the memo.

Prosecutors also said Durham's actions have had a chilling effect on the lives of dozens of foreign volunteers in Kenya and elsewhere "who must now live under the cloud of suspicion, distrust and apprehension when they volunteer their time, talent and resources for the betterment of children in East Africa and beyond."

"There is a real perception among Upendo's local Kenyan community that more pedophiles lurk among the volunteers, especially the young male volunteers," prosecutors said.

Evidence produced by prosecutors included handwritten, signed confessions that Durham gave orphanage officials after he was accused of inappropriate behavior.

Jones has argued that the statements were coerced by orphanage officials who isolated Durham, took his passport and created the allegations to obtain $17,000 from the U.S. government for security cameras.

Jones has described Durham, who was 19 when he was charged in 2014, as "an emotionally vulnerable teenager" who was struggling with "sexual identity and development" while also being a devout Christian.

Associated Press