Police have slapped more charges on a youth accused of killing Indian student Abhijit Mahato, as shocked family members and students held a prayer service for the 29-year-old Duke University alumni.

Police have charged murder suspect Stephen L Oates and his friend William Dozia Smith with contributing to the delinquency of teenagers they were arrested with in an armed robbery case, according to court documents.

Smith, 20, and Oates, 19, were each charged with eight counts of contributing to the delinquency of a minor. Warrants say the two men caused and encouraged the boys to be at the scene of three armed robberies and in a car speeding to elude police.

Mahato, a PhD student at Duke's Pratt College of Engineering in North Carolina, was gunned down on January 25. Duke police said Oates shot and killed him during a robbery spree.

Meanwhile, family members, friends and Duke University officials paid respects to Mahato by holding a prayer service at a Hindu temple in Morrisville, a town neighbouring Durham, where the Indian student lived.

At the prayer service, Mahato's friend Rinku Majumder was still trying to get past the memory of discovering Mahato's body.

"We went inside and we saw his body, with lots of blood," said Majumder.

"He won't come back no matter what, but at least I want to know why you should kill him," he said.

Tod Laursen, a Duke professor, praised Mahato, saying, "He was interested in developing relationships with other very bright students all over the world. He was interested in the way he would grow."

Some mourners couldn't fight the tears. "He always used to tell me to keep laughing, that is it," a teary-eyed Tanmay Patni said.