SAN DIEGO (AP) — A flesh-eating bacteria linked to the use of black tar heroin has killed at least seven people over the past two months in San Diego County, authorities said.

Nine people who injected black tar heroin between Oct. 2 and Nov. 24 were hospitalized with severe myonecrosis, a soft-tissue infection that destroys muscle, county health officials said Wednesday.

Of the seven who died, five were men. The nine patients ranged from 19 to 57.

Investigators are trying to determine the source of the heroin.

Officials have advised the local medical community to watch for additional cases of myonecrosis and wound botulism, a rare but serious illness that attacks the body’s nerves and is also linked to black tar heroin use.

Symptoms of myonecrosis include pain, swelling, pale skin, blisters with foul-smelling discharge, fever, excessive sweating and increased heart rate.

If left untreated, myonecrosis can spread through the body and cause people to go into shock. It can lead to amputations or death.

Symptoms of wound botulism can sometimes be mistaken for a drug overdose and occur within days or weeks of injecting contaminated drugs. Symptoms can include drooping eyelids, blurred vision, dry mouth, sore throat, slurred speech and paralysis.

Left untreated, symptoms may lead to paralysis of the respiratory muscles, arms, legs and torso and can cause death.

Black tar heroin is sticky like roofing tar or hard like coal and is predominantly produced in Mexico and sold in U.S. areas west of the Mississippi River, according to the National Institute on Drug Abuse. The dark color results from crude processing methods that leave behind impurities. Impure heroin is usually dissolved, diluted, and injected into veins, muscles or under the skin.

A 2016 survey by the Drug Enforcement Administration found black tar heroin sold in 12 Western cities, including San Diego, Los Angeles, Phoenix and Denver, nearly all of it from Mexico. Purity ranged from 34% to 43%.