The juvenile arrested on suspicion of first-degree murder in the death of Metro Taxi driver Darren Bloomquist is not being formally charged at this time.
The Denver District Attorney's Office told 7NEWS prosecutors were facing a court deadline to file charges.
There was enough probable cause to arrest the juvenile, but the burden of proof was not met for charging him, the district attorney's office said.
However, the teenager is not being released at this time. Officials said the teen will remain in custody for an unrelated case.
The teen's name, age and criminal background have not been released because he's a juvenile.
Bloomquist was shot and killed inside his taxi on Sept. 5, after receiving a call to pick up a fare in the Green Valley Ranch neighborhood.
Bloomquist, 49, worked for Metro Taxi for 10 years before he was shot at 2000 East Mitchell Place. Police found him dead a few blocks away.
Metro Taxi said Bloomquist lived a selfless life, even serving his country as a member of the Air Force. He was moonlighting as a cab driver to support his wife, who Metro Taxi described as physically challenged.