SAN FRANCISCO (CBS SF) — The mother of the young man fatally shot while playing Pokemon Go in a San Francisco park nearly a year ago made a tearful plea on Monday for witnesses to step forward.

The man’s family and authorities are offering an increased reward to motivate possible witnesses and keep the case from going cold.

Calvin Riley, a 20-year-old college student, was playing the popular mobile game Pokemon Go with a friend in San Francisco’s Aquatic Park on August 6 of last year when the fatal encounter happened.

Just before 10 p.m., he was shot and killed. There was no confrontation, no robbery and no apparent motive for the shooting.

Officials held a press conference near the scene of the crime with Riley’s family Monday morning to announce the $110,000 reward is on the table for information leading to the killers arrest.

$50,000 came from the Riley family. An additional $50,000 comes from the city of San Francisco along with $10,000 from the U.S. Park Police.

“It’s so important to our family to have closure, to find the person who shot my baby boy in the back,” said Calvin’s mother, Kariann Riley.

One of the few clues in the case is a sketch of a male suspect seen running from the scene.

“We have interviewed countless folks of associates and friends and at this time, we just haven’t found someone with the motive to kill him. although we have targets that I can’t comment on at this time,” said U.S. Park Police Captain Jerry Marshall.

Congresswoman Jackie Speier, who represents the city of San Mateo where Riley grew up, also spoke at the press conference.

“We do know that money motivates people,” said Speier. “The person who did this crime must be brought to justice.”

Police have identified two types of vehicles that may have been associated with the crime: a black Audi A3, model year 2006 to 2009 and a white 2013 4-door Hundai Sonata.

Suspect vehicles in SF Pokemon Go slaying (U.S. Park Police Department)

Riley was attending Delta College in Stockton and had graduated from Junipero Serra High School in San Mateo.

His family says the past year has been hell.

“We don’t know how to go on without him,” said Kariann Riley. “We try our best every day to make sense and there’s no sense in it.”

Anyone with information is asked to call the U.S. Park Police. Witnesses have the option to remain anonymous.