Authorities say Richard Gonzalez, an 18-year-old Fontana resident, was found dead in his jail cell at the West Valley Detention Center in Rancho Cucamonga just before 7 p.m. Wednesday.
The San Bernardino County Sheriff's Department says he hanged himself with a bedsheet. He and his friend are both charged with murder.
Gonzalez and two others were scheduled to be arraigned Thursday morning.
Fontana police say Morales was the ex-boyfriend of the victim's daughter, who the victim had told repeatedly to stay away.
"It sounds like they went there to rob him, and they ended up kidnapping him and killing him," said Sgt. Doug Imhof.
Morales' mother was charged as accessory to murder. His 17-year-old brother will also be charged with accessory to murder.
Detectives arrested Edward Anthony Morales and Gonzalez on suspicion of murder on Tuesday. Morales' mother, 37-year-old Alma Quezada of Fontana, was arrested as an accessory to murder.
Alejandro Padilla, 40, disappeared after leaving for work early on the morning of July 26. He was found dead later that night in Rialto in his burned car.
Investigators believe the suspects went to Padilla's house with the intent to rob him, but they ended up killing him. Morales had formerly dated Padilla's daughter and knew the victim and his family, according to police.
A fourth suspect, a juvenile, has also been taken into custody as an accessory to murder.