CENTENNIAL, Colo. -- Police said they are putting together the final moments of 23-year-old Andrew Gleston Graham's life to better understand who killed him and why.

Graham was found face-down on a Centennial lawn last week. He bled to death from a single gunshot wound. Authorities have no suspects and no leads.

"We have obtained some video from the light rail system that runs along I-25," said Arapahoe County Sheriff Grayson Robinson.

Robinson said the video shows Graham boarding the light rail at Denver's Union Station around 11 p.m on Thursday, Nov. 5.

Two couples and two women, who appear to be friends, were also on the train but they did not seem to have any interaction with Graham.

Robinson said investigators are interested in talking to the six people on the train but stressed they are not suspects in the case.

"These six people are the last people that we know of that were in his presence while he was still alive," said Robinson. "We do not believe any of the six were involved in his death but we believe any one of the six could have information that could be instrumental or critical to the investigation."

Robinson said Graham left the RTD station at I-25 and County Line Road around 11:40 p.m. What happened to Graham after that is a mystery.

Graham's body was found in front of a home in the 8700 block of East Phillips Place, about a mile from his parent's home.

Neighbors said they heard gunshots around 12:15 a.m. and investigators said they would follow up on that lead this week.

"It is our intention to go back into that neighborhood and try to triangulate where residents think they heard a gunshot," said Robinson.

Graham's parents are beside themselves looking for answers.

Graham had spent the day in Boulder looking for a place to stay while he started his graduate's degree in engineering.

Police do not believe Graham had any enemies and the belongings in his satchel, including a wallet and passport, were all intact.