The suspect's girlfriend, Chelsea Thompson, told police that after the shooting her boyfriend called her crying, admitting to opening fire on two people, the prosecutor testified.
Thompson told police her boyfriend had recently been diagnosed with a personality disorder and battled substance abuse.
Michael Close has been arrested and charged over the attack.
Following Thallas' death, a GoFundMe was set up which has raised more than $100,000 for funeral costs.
"My heart just goes out to the family, just sending lots of love and prayers and care to the family at this time," said family friend Priscilla Rahn, who created the GoFundMe page, according to CBS Denver.
"We just want to wrap our arms around the family and help support and relieve some of that stress."
Close was also crying and apologising when he was arrested, it was reported.