Four teenagers were charged as adults Friday with attempted murder for allegedly beating and stabbing a homeless man last month.

The four teenagers, who range in age from 15 to 17 and live in Fort Lauderdale, had been charged with aggravated battery as juveniles for the Sept. 20 attack on 44-year-old William Teeters, who was left bleeding and bruised after the attack at a local park.

The Broward County State Attorney's office upgraded the charges after reviewing the evidence.

"The evidence we've collected reveals an extreme callousness and recklessness. It's extremely disturbing," said Maria Schneider, a prosecutor who heads the state attorney's juvenile unit.

Bobby Callins, 17, was arrested Thursday after investigators found additional evidence linking him to the crime, said police Detective Katherine Collins.

The three 15-year-olds who were already in custody were identified by police as Michael Livingston, Patrick Keels and Romel Jean-Louis.

It was not immediately clear if the teens had lawyers.

They were identified from a surveillance video that put them within a quarter-mile of the park 10 minutes before the attack, police said. Authorities had said the boys were considered "persons of interest" because they matched Teeters' description of the attackers.

Teeters was attacked near the site where another homeless man, Norris Gaynor, 45, was beaten to death by a group of teens in January. Three teenagers are charged with murder in that case.