The ex-boyfriend of an 18-year-old recent high school graduate in Morgan Hill who was found dead last week was arrested Sunday as he tried to cross back into the United States from Mexico.

Martin Mendoza, 20, was taken into custody at the southern border Sunday afternoon, and he is now in jail in El Centro, California, as the Morgan Hill Times reports. Mendoza is considered a person of interest in the death of 18-year-old Marissa DiNapoli, who was found dead last Wednesday after being missing for several days. DiNapoli had last been seen alive on June 29, in the company of Mendoza.

DiNapoli's body was found on July 2 near the Anderson River reservoir. She had previously been in a romantic relationship with Mendoza.

A friend of DiNapoli's told NBC Bay Area last week that she believes Mendoza was likely her killer, and that her friend had told her "that [Mendoza] would kill her, that he would not stop, that he’s obsessed with her, that he’s crazy and that he’s willing to go to that extent."

Morgan Hill police had quickly identified Mendoza as a person of interest after DiNapoli was reported missing, and they said he was uncooperative when they first interviewed him. Mendoza then disappeared after that first interview and they could not locate him — and we now know why.

As KTVU reports, there were 30 investigators working on this case at one point, and the FBI came to the assistance of Morgan Hill police.

No motive for the crime has been discussed, and we don't yet know the cause of death.

DiNapoli's family said they had last spoken to her on Saturday, June 28.

Previously: Body Found in Morgan Hill Park Identified as Woman Who’s Been Missing for Five Days