The photog who was killed taking pics of Justin Bieber's car made a call less than an hour before he died, telling someone he was on a big story -- because he claimed he saw Justin smoking marijuana while driving.
A source connected with the photo agency for whom Chris Guerra did freelance work tells TMZ ... Chris called him at 5:16 Tuesday afternoon. Chris said, "Man, you won't believe my luck. It's pretty dead in L.A.. Everybody thinks Justin and Selena are in Mexico, but they're not."
Chris -- who worked exclusively on the Bieber detail -- said he had spotted Justin's car earlier in the day at the Four Seasons Hotel in Beverly Hills and also saw Selena Gomez's car there.
Chris went on ... claiming he saw Justin earlier in the day smoking what he believed was marijuana from a pipe while driving the Ferrari.
Our source says Chris told him he was following Justin back to the Four Seasons and was continuing to pursue what he believed was a big story.
The source says after 20 minutes, Chris abruptly said the Ferrari was pulling out of the hotel parking lot and he needed to hang up and follow it. Apparently he had no idea Justin was NOT in the car.
Our source -- who as we said is connected to the photo agency -- believes Chris was probably "overzealous" at the traffic stop because he believed he was on the cusp of an enormous scoop.
Justin's rep tells TMZ, "It's really sad that people are trying to push a story without the facts so soon after this tragic accident. Justin was not present or involved in this incident and the focus should remain on honoring the memory of the victim."
The rep did not directly respond to the marijuana allegation.