Three men found dead in San Francisco’s Haight-Ashbury neighborhood Thursday had trace amounts of the lethal opioid fentanyl in their bodies, city officials said Friday, reinforcing the belief that adulterated street drugs caused their deaths.

The Department of Public Health said the men were likely using methamphetamine augmented with fentanyl and the fentanyl-like substance, acetyl fentanyl.

Fentanyl and fentanyl-like compounds are synthetic opioids commonly mixed with street drugs. Because of their strength, they’ve been tied to tens of thousands of overdose deaths in the United States in recent years.

While an analysis by the Medical Examiner’s Office showed fentanyl in the victims’ bodies as well as in the drug paraphernalia they possessed, officials said they would not be able to confirm the causes of death until additional toxicology tests are completed.

The men were identified as Michael Campbell, 32, David Clark, 31, and Adam Wilson, 36. The residency of the victims was not immediately known, though authorities said they did not live in the neighborhood.

A school security guard came across the victims at 4:35 a.m. on the 1500 block of Page Street. Authorities said there were no signs of foul play or weather-related distress.

The deaths prompted public health officials to issue a warning to drug users to be on the alert for fentanyl-laced narcotics. Several San Francisco syringe-access sites offer fentanyl test strips.

The number of deaths related to fentanyl has been on the rise in the city. In 2016, 22 people died from the drug, up from 11 in 2015, according to public health officials. Over these two years, 45 percent of those cases involved fentanyl mixed with methamphetamine, 27 percent with cocaine and 41 percent with some other opioid.