Emergency medical officials released an alert Friday, warning of a potentially deadly batch of heroin being sold in Massachusetts.

"There is a very dangerous batch of heroin circulating the streets of Greater Lowell," Trinity EMS, an emergency transportation company based in the Greater Merrimack Valley, said in a statement Friday.

Emergency medical technicians from the company were called to a half-dozen overdoses in a 15-hour period this week. "All 6 patients were critical," the statement released by Trinity reads.

Nearly 2,000 people in Massachusetts died from opioid overdoses in 2016, a 13 percent increase over the previous year's fatalities.

The cause of fatalities has shifted from heroin to the synthetic opioid Fentanyl, a drug that can be 50 to 100 times stronger than heroin and is often mixed with or substituted for heroin. Fentanyl contributed to 75 percent of fatal opioid overdoses in Massachusetts last year, according to the state Department of Public Health.

For every fatal overdose, there are between 10 and 12 non-fatal ones, health officials estimate.