Doctoral student killed by stray bullet in Chicago just hours after arriving to study at Northwestern

A 25-year-old doctoral student who had just arrived in Chicago was killed by a stray bullet on Sunday night.

Shane Colombo, a California native who had studied at Columbia University in New York City the last two years, was shot near a bus stop in the Rogers Park neighborhood on the far north side of the city. He had been scheduled to join Northwestern's psychology program.

"We Skyped and I told him like, 'you need to stay home. You need to rest. You just got in,'" his grieving fiance Vincent Perez said during a press conference on Tuesday.

Colombo, a child cancer survivor, and Perez were going to live together in Chicago.

"I was very concerned about him coming out here, and he was killed within four hours of being in the city, four hours of stepping off that plane," mother Tonya Colombo told ABC News. "I put him on a plane that morning at 10 a.m. [in California] and I kissed him goodbye, and that was the last time I saw him alive."

Investigators say Colombo was caught in the crossfire as two suspects fired at each other. He was shot in the abdomen and passed away at a nearby hospital.

"I got worried because he wasn't responding to my text," Perez said. "I checked his location on my phone because I had his location, and it showed that he was in the hospital. I called and they told me he had just passed. So I didn't get to say goodbye to him."

The Chicago Police Department released surveillance images of three suspects who cops say ran away from the scene.

"The slightest bit of information can help detectives with this investigation," police spokesperson Anthony Guglielmi tweeted.

Colombo's mother told ABC News that her son had set out to buy clothes hangers for his new home when he was shot.

"He was a bright, beautiful, amazing son. And he was so loving," she told the network.

Colombo had been doing research in the Social Cognitive and Affective Neuroscience Lab at Columbia. His mother said he always pushed himself despite missing a year of high school while battling lymphoma.

A GoFundMe page is raising money for his family.

"He was so passionate about what he was doing," Tonya Colombo told ABC News. "He was going to be a doctor. He wanted to do clinical research. He wanted to give people answers."

A total of six people were fatally shot in Chicago during the Labor Day weekend.

• With News Wire Services