The death of a man in Denver’s city jail came after what is believed to have been a two-punch fistfight that was over in seconds, 9Wants to Know has learned.

William Anderson, 42, died a little after 7 a.m. Monday after throwing a punch, then being hit once and falling face-first to the floor in a fifth-floor shower at the downtown jail, according to a source familiar with the investigation.

No criminal charges will be filed against the 19-year-old inmate suspected of hitting Anderson, prosecutors announced in court Tuesday. 9NEWS is not naming him because he was not charged with a crime related to this fight. He’s also no longer in jail.

Surveillance video from inside the jail showed that a sheriff’s deputy was moving to stop the confrontation within two seconds of it starting – and that the entire episode was over in roughly four seconds, the source told 9NEWS.

The trouble erupted in the 5H pod at the Van Cise-Simonet Detention Center, 490 W. Colfax Ave. The area includes a day room with cells arranged along it on two floors.

The incident began, investigators have concluded, when Anderson began yelling in the common area and another inmate in an upper-floor cell said something in response. After words were exchanged, the two men headed to a second-floor shower – a common place for fights in the jail because there are no cameras.

It was a physical mismatch – at least on paper.

Anderson’s arrest record, which covers six pages and dates back more than two decades, lists him at 6-foot-4 and 190 pounds. Court records describe the other inmate as 5-foot-6, 125 pounds.

According to the source, it’s believed that Anderson threw the first punch and the other inmate – standing on the lip of a shower – responded by hitting the man back. Anderson fell face first to the floor and died a short time later despite resuscitation efforts carried out by deputies, firefighters and paramedics.

The exact cause of his death has not been determined.

The death is the subject of an internal investigation, 9NEWS has confirmed.

According to court and Colorado Bureau of Investigation records, Anderson had been arrested more than a dozen times since 1994 on charges including theft, assault, drug possession and distribution, and failure to appear. He had been picked up most recently July 1 after an incident at East Colfax Avenue and Emerson Street. He was charged with assault.

The other inmate had been cited multiple times on traffic charges, including an arrest for driving while under the influence of alcohol in 2015.

Most recently, according to court records obtained by 9NEWS, he was charged with eluding a police officer, driving without a license, driving without insurance and driving while his license was under restraint after an incident in March in which he allegedly turned in front of a Denver police officer, nearly causing a crash, and then sped away. Records show he was jailed June 8 on those charges.

Contact 9NEWS reporter Kevin Vaughan with tips about this or any story: or 303-871-1862.