The ex-boyfriend of murdered California teen Marissa DiNapoli has been arrested while returning to the US from Mexico — as a new video shows she was at his home the last time she was seen alive.

Martin Mendoza Jr., 20, was detained Sunday after being named a person of interest in the murder of his 18-year-old ex, whose body was found in a park in Morgan Hill, KSBW reports.

DiNapoli had told friends that Mendoza was “obsessed” with her and so “crazy” that he “would kill her,” one friend has said.

The last time the teen was seen alive, June 29, she was at Mendoza’s home in Morgan Hill, with Ring camera footage showing them walking down a staircase together, according to footage obtained by KSBW.

The following day, Mendoza was seen leaving the house in the wealthy Bay Area town carrying a plastic bag, the video shows.

DiNapoli was reported missing the same day — and her body was found two days later near Anderson Lake.

A major manhunt was launched for her ex, involving more than 30 investigators from 15 law enforcement agencies.

Mendoza was finally arrested Sunday as he tried to re-enter the US through Calexico, California, KSBW reported.

He did not give a statement to police, authorities said.

No cause of death has been issued for DiNapoli, and the motive is unknown, police said.

“Marissa was taken from us way too young,” said Mario Ramirez of the Morgan Hill Police Department. “With this arrest, I think it helps the family be able to move forward.”

DiNapoli had expressed her concerns that Mendoza would harm her, a friend, Jaslyn Gutierrez, previously told NBC Bay Area.

“Marissa has told me, firsthand in the car with my friends outside of my house, that he 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,” said Gutierrez.

Mendoza was transported back to Santa Clara County on Monday and is due in court Tuesday afternoon.

He is being held on no bail in the Santa Clara County Main Jail, court records indicate.