MEMPHIS, Tenn. (AP) — A U.S. attorney in Tennessee is joining other federal and state attorneys who are urging Congress to extend an order that criminalizes possession of the dangerous opioid fentanyl and related substances.

D. Michael Dunavant, the U.S . attorney for West Tennessee based in Memphis, said in a statement Tuesday that law enforcement would lose an important tool for fighting illegal fentanyl distribution if the order is not extended or made permanent. Fentanyl is a synthetic opioid that is considered much more dangerous than heroin or methamphetamine.

Heroin laced with fentanyl has led to overdose deaths that have been prosecuted by Dunavant's office and other federal jurisdictions around the country.

In February 2018, the Drug Enforcement Administration issued an emergency two-year order that made all fentanyl-related substances illegal. That order expires Feb. 6.

In December, all 56 state and territorial attorneys general issued a letter calling for passage of a proposed c ongressional act that makes fentanyl and related substances illegal on a permanent basis.

U.S. attorneys in New England states and Utah also have made similar calls for c ongressional action on the order, which has allowed law enforcement to arrest and prosecute those who make or distribute fentanyl-related substances.

In his letter, Dunavant said authorities in West Tennessee, a region that includes Memphis and some smaller rural cities and towns, are seeing increasing amounts of fentanyl produced by Mexican cartels, in addition to the relatively small amounts of the drug from China that they were seeing in the region previously.

In November, authorities in Ohio intercepted more than 44 pounds (20 kilograms) of fentanyl, plus meth and heroin, according to the December letter from the National Association of Attorneys General. Testing showed that the fentanyl was laced with carfentanil, which can be over 100 times more potent than fentanyl, the letter said .

“The potency of the fentanyl-related substances in this single bust was enough to kill every man, woman, and child in the state of Ohio several times over,” said the letter, which was addressed to Republican Sen. Lindsey Graham of South Carolina and Democratic Sen. Dianne Feinstein of California.

In November, a man from Jackson, Tennessee, was sentenced to 15 years in federal prison for possessing fentanyl and heroin with intent to distribute. A 32-year-old Memphis man was sentenced in March to 20 years in prison for selling fentanyl that resulted in an overdose death.

"We need every tool we have to target this dangerous drug, hold dealers accountable, deter others from selling poison to our citizens, and save lives," Dunavant said. “Congress must take action immediately.”