(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, went missing 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 September 20. The second teen-ager 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.

The man has not been charged, but authorities have said he is a "person of interest" and ordered him held for psychiatric evaluation after he told investigators that he wanted to kill himself, Officer Ben Tobias said Monday.

Police suspect foul play, Tobias said.

Family members who had already plastered the area with fliers and even took 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.

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