FORT WORTH (CBSDFW.COM) – Thomas Harper seems to be a man without a past.

The 28-year-old has no record of a driver’s license, no job and no indication that he’s ever committed a crime.

Tarrant County Sheriff Dee Anderson says Harper has been a model prisoner, but they wanted to know more about him.

“We ended up today, in fact, talking to his father on the telephone he still lives in Mississippi,” says Sheriff Anderson “It’s kind of unusual when you see someone that is 28 years old who has some large gaps in his background.”

Anderson says Harper’s father filled in some of those gaps. He told investigators that Harper has lived in various cities with relatives over the years.

He also talked about Harper’s erratic behavior and struggles with mental health issues.

“He did say he had some issues and problems to the point where he did get on medication for that for some length of time,” Anderson said. “We asked if he had been institutionalized for serious trauma – anything similar to what we saw –  he denied that.”

Last Wednesday, Harper, with his twin toddlers in the SUV, took off from a hit and run in Arlington, investigators say.

He’s accused of plowing into a pickup truck, killing Najee Nasir, 42.

Police say moments later Harper shot and killed 18-year-old CJ Robinson, who walked up to the wrecked SUV after the crash to help.

“He was supposed to work all this week, he was supposed to be here with me today right now,” says Tangie Robledo who is a friend and co-worker.

At the Potbelly sandwich shop not far from the scene, friends and co-workers are mourning.

They say Robinson will be buried Friday in New Orleans and they, like investigators, want to know what happened.

“I’m angry that it had to happen in the first place. So now learning that he had issues – mental issues and people were aware and maybe it was mismanaged it makes me even more angry, because it was all unnecessary,” said Kesha Riley, who hired Robinson at the Potbelly.

According to an arrest warrant, Harper did have marijuana in his system.

His father told investigators his son has never been violent and he still doesn’t have answers after talking to him on the phone.

“His father said he’s received several phone calls and they seemed normal and he seemed concerned and confused about what’s going on and expressed his desire to come home,” Anderson said.

His father also told investigators that Harper’s medication helped him. He’s not sure if Harper had been off his medication leading up to what happened in Arlington.

Anderson is also checking to make sure Harper didn’t have any aliases.

He’s currently charged with murder, manslaughter and endangering the lives of his toddler twins.

He is under suicide watch at the Tarrant County Jail and has declined requests for an interview.