Here are things to know about the synthetic-opioid epidemic in US

Washington: Government scientists say they're seeing the emergence of a new class of deadly drugs built to mimic the potent prescription painkiller fentanyl. What you need to know:

Dozens of variants

The U.S. Drug Enforcement Administration's Special Testing and Research Laboratory has around 50 fentanyl-related compounds in its drugs database. Regulators are trying to catch up. China has blacklisted 19 fentanyl-related molecules and is now considering adding four more to its list of controlled substances, including the weapons-grade carfentanil .

Easy to order

It's easy to source synthetic opioids online from Chinese vendors. Associated Press reporters identified More than a dozen companies offer to export fentanyl-like drugs. Some say they'd lie on customs forms, disguise packaging and even guarantee delivery. After Chinese officials warned vendors that one analog, carfentanil, is hazardous and shouldn't be sold, vendors started marketing other opioids, like U-47700.

Dispute over source country

U.S. officials say casework and investigations show China is the main source of fentanyl-related drugs. But no U.S. agency has been able to produce comprehensive statistics. China says U.S. claims are unsubstantiated and has asked for more evidence.

Mexican connection

Mexican drug cartels are profiting from the opioid trade. Nearly two-thirds of the 61 kilograms (134 pounds) of fentanyl seized by U.S. customs last year came from Mexico. Mexican authorities raided seven fentanyl labs in 2014, five of them in Sinaloa state, according to the office of Mexico's attorney general. Mexican and U.S. authorities say those labs mainly use material imported from China to make the drugs.

Growing demand, and deaths

The proliferation of synthetic opioids feeds U.S. demand. Last year, U.S. authorities identified derivatives of fentanyl nearly 2,000 times, up from just 78 times in 2014. Unsuspecting users have been overdosing in record numbers on these poorly understood new chemicals, which are often cut into heroin or counterfeit pills. Deaths from synthetic opioids, including fentanyl and related molecules, more than tripled from 2013 to 2015, hitting 9,580 last year.