(CNN) -- Nearly 100 people, including police, volunteers and family members, were searching Monday for a University of Florida student who went missing four days ago in Gainesville.
Police fear Christian Aguilar could be seriously injured, disoriented and unable to seek medical attention.
Aguilar, 18, disappeared last week after an altercation over a girl, his father told CNN affiliate WCJB.
According to police, Aguilar and another 18-year-old man had an altercation on Thursday. The second man told investigators he left Aguilar in the parking lot, but searchers using helicopters and scent dogs failed to turn up any trace of him, police said Sunday.
Authorities have said the man is a "person of interest" in the case and charged the 18-year-old with depriving a victim of medical care, a third-degree felony. He underwent a psychiatric evaluation Monday after he told investigators that he wanted to kill himself and was subsequently booked in the Alachua County Jail, Officer Ben Tobias said.
Police suspect foul play, Tobias said.
Family members, who have plastered the area with fliers and taken to Twitter to generate leads, have invited a nonprofit search group to help look for Aguilar alongside family members and police.
"The search is ongoing. We're going to keep searching until we find him," said Joe St. Cyr with Search and Rescue of Central Florida.
CNN's Joe Sutton contributed to this report.