Investigators say Joseph Leo Amaya sexually assaulted the college student as she was closing the Peck Mini Mart in the 4100 block of Peck Road.
According to police, Amaya blocked the exit and then dragged the young store clerk to a back room and raped her.
El Monte resident Marcelana Tapia heard the victim's screams after the attack. Tapia's son ran outside to help. She says he recognized the victim having just been inside the convenience store earlier that night.
"If he would have gone 20 minutes after, he probably would have seen when it was happening," said Tapia.
Police say the victim had never seen Amaya before the attack, which was captured by surveillance cameras. Those images, along with Amaya's penchant for tattoos, helped detectives identify him as the suspect.
"We think the subject has an identifying tattoo on his neck. We are absolutely positive he has a tattoo on his right arm and it has the initials 'EMF' written on it," said Sgt. Roger Cobian with the El Monte Police Department.
"EMF" is the common tag of the street gang El Monte Flores.
Investigators say Amaya has a criminal history and many ties to El Monte. They fear someone may be helping him hide. They are asking for the public to turn him in.
"Our detectives are working exhaustively to find this subject. We have not had enough leads to be able to get him into our custody. We are now asking for the public's help," said Cobian.
Police say Amaya is 5 feet 5 inches tall and weighs about 160 pounds. They say he is considered dangerous, so if you see him, don't approach him.
Anyone with information was asked to call Los Angeles Regional Crime Stoppers at (800) 222-TIPS.