Brett Michael Olson, a 20-year-old from Lafayette, Calif., was last seen Sunday afternoon while participating in a Labor Day weekend celebratory float down the Sacramento River near California State University, Chico.

A Facebook group called “Let’s Bring Brett home," created Tuesday evening, had nearly 65,000 members as of Thursday night and has been used for everything from organizing search efforts to sharing stories and supportive messages.

According to a post on the group by Michael Conrad Bissell, a University of Oregon student and a high school classmate of Olson’s who was with him on Sunday, some of the last people to see Olson at the float may have been a group of University of California, Los Angeles students. Bissell said that they all got off their inner tubes at an area known as Beer Can Beach and that Olson had disappeared soon after they exited the river.

Members of the group have taken to Twitter as well, using the hashtag #FindBrettOlson in order to spread the word. The message has so far received support from celebrity Twitter users including Olympian Kerri Walsh-Jennings and television personality Howie Mandel.

#Findbrettolson speed the word, please. Young man from Chico area has been missing since Sunday. Please send... fb.me/20HXweDua

— Kerri Walsh (@kerrileewalsh) September 6, 2012

Please retweet RT @bronwynwarzenia: Hope you're safe Brett!! Everyone can't stop thinking about you! #FindBrettOlson@howiemandel

— Howie Mandel (@howiemandel) September 7, 2012

Crews have searched areas where Olson was likely to be found using aerial heat-sensing technology, NBC Bay Area reported. California State Parks also is conducting a land search. ABC7 News reported that law enforcement teams used cadaver dogs to search for human remains in the area with no results.

Olson is 6’1” and about 170 pounds with hazel eyes and short dark brown hair. He was last seen wearing purple and white patterned board shorts. Anyone with information is asked to contact Glenn County Sheriff’s Office at 530-934-6441.

Reach contributor Jared Servantez here.