Synthetic marijuana is suspected to be behind an illness that has sent dozens of homeless people from a downtown Los Angeles neighborhood to emergency rooms in recent days, authorities said.

Eighteen people from the Skid Row homeless district were treated by firefighters on Monday and 14 were taken to hospitals, Los Angeles Fire Department spokeswoman Margaret Stewart said.

That came after the department on Friday evaluated about 50 people in the neighborhood, transporting 38 to hospitals.

Although the cause of the illnesses has not yet been confirmed by toxicology tests, authorities said it was likely a type of synthetic marijuana called "spice."

Local television news showed multiple men passed out in the street, many of them being treated by firefighters or other health personnel.

"Because it's synthetic, nobody for sure knows what active ingredients are in there, the strength is variable," Marc Eckstein, the Los Angeles Fire Department medical director, said in a statement.

He told local television station KTLA that the problem is a "public health crisis" in Los Angeles, home to the largest population of homeless people in the United States without access to emergency shelter.

Some 5,000 homeless people -- many with psychological and addiction problems -- live in Skid Row, mostly sleeping in the streets.