BOGOTA, Colombia — Colombian authorities said that a one-armed explosives expert belonging to the country’s last remaining rebel group carried out a car bombing against a police academy that left 21 dead and dozens more wounded. Chief Prosecutor Nestor Martinez said at a press conference Friday that the man who carried out Thursday’s brazen attack, the deadliest in 15 years in the South American nation, was a longtime member of the National Liberation Army, or ELN. Even though Jose Aldemar Rojas had no criminal record, Martinez said that the 56-year-old man is the same individual known in intelligence reports by alias Mocho Kiko, who lost his arm manipulating explosives during a long clandestine career with an ELN cell near the border with Venezuela. The shock bombing recalled some of the bloodiest chapters of Colombia’s recent past and has raised tough questions about lingering security threats in the wake of a 2016 peace deal with FARC rebels.