"You're a coward that, you took the most precious person in our lives," said Kariann Riley, trying to hold back tears as they welled-up in her eyes.

Kariann Riley and husband Sean are making an emotional plea to find their son Calvin's killer.

"It's been a nightmare every single day. It's torn our family apart. It's a living hell," said Calvin's father Sean Riley.

Their 20-year-old son Calvin was a beloved pitcher on Stockton's San Joaquin Delta College baseball team.

But, last August 6th around 10 at San Francisco's Aquatic Park, Calvin was playing the popular Pokemon Go game.

Out of nowhere, a bullet struck him in the back and killed him.

"We have done countless interviews with family and friends and others and we have found no one who has any type of animosity or any bad feelings toward Calvin," said U.S. Park Police Captain Jerry Marshall.

In front of the ball field at San Joaquin Delta College where Calvin took to the mound as a pitcher, the U.S. Park Police pointed no witnesses, no motive.

“To shoot somebody in the back from 15 to 30 feet away is a gutless coward act," said Sean Riley.

“Every second we think of him," a tearful Kariann Riley said.

“What Cal loved most was baseball. He loved the competition it brought. But he loved his teammates and the relationships the game brought more than anything else," said Delta College baseball coach Reed Peters.

Peters said before each practice, Delta players come to the center field wall and touch number 27, Calvin’s number, surrounded by a giant green clover in his memory.

On this day, many of Calvin’s former teammates were on hand to lend their support to Calvin’s mom and dad.

“Calvin was a once in a lifetime kid. He was just someone who brought energy to everyone around him. He was always a positive person. He was really loving," said Alex Patterson, who is now a pitcher at UC Santa Barbara.

“Unmatched person I have ever met. He was a phenomenal competitor as a baseball player, but just to describe him as a phenomenal baseball player wouldn't do him justice because he was just an awesome human being," said Joey Skracic, a Bakersfield native, who now pitches for Campbell University in North Carolina.

The family hopes by keeping their son’s memory alive, it will also help catch a killer and provide some closure for a son who prized his Irish and Boston area roots.

“Just an amazing person. You stole him from us. You stole him," said Kariann.

There are two vehicles of interest in the case.

One is a dark Audi A3 Wagon.

The other vehicle is a 2013 white 4 door Hyundai Sonata.

If you have any information, you are urged to call the United States Park Police Tip Line at (415) 561-5150.

There is a $110,000 reward for information leading to an arrest or conviction of the person or persons responsible for the crime.