MEXICO CITY (AP) — Banners appeared Monday in northern Mexico purportedly signed by a faction of Mexico's Sinaloa cartel claiming that the gang has sworn off the sale and production of the synthetic opioid fentanyl.

But experts quickly cast doubt on the veracity of the claim, saying that fentanyl — which has caused tens of thousands of overdose deaths in the United States — remains one of the cartel's biggest money makers.

Prosecutors in Sinaloa confirmed that the banners appeared on overpasses and near roadways, but could not say whether they were authentic or who had hung them up.

The machine-printed banners purportedly signed by the sons of imprisoned drug lord Joaquin "El Chapo" Guzman claim they have prohibited the sale or production of fentanyl in the northern state of Sinaloa. The sons are known as "the Chapitos" after their famous father.

"In Sinaloa, the sale, manufacture, transport or any other business dealing with fentanyl, is strictly prohibited, including the sale of chemicals used to produce it," the banners read. "You have been warned. Respectfully, Chapitos."

Mike Vigil, former head of international operations for the Drug Enforcement Administration, said there is concrete evidence that "Sinaloa is the biggest producer of fentanyl in Mexico" and that there has been no sign the cartel is moving away from it.

"I think the Chapitos started feeling the pressure when they increased the reward for their capture. I think they are trying to create a massive illusion to take the pressure off," he said. "It's almost like a big campaign to convince the U.S. they're not involved. It's nothing more than pure propaganda," Vigil said.

In September, Mexico extradited Ovidio Guzmán López, one of the Chapitos, to the United States to face drug trafficking, money laundering and other charges. Mexican security forces captured Guzmán López, alias "the Mouse," in January in Culiacan, capital of Sinaloa state, the cartel's namesake.

In May, the Chapitos claimed in a letter that they were not involved in the fentanyl trade. The sons of Guzmán wrote at the time that "we have never produced, manufactured or commercialized fentanyl nor any of its derivatives," the letter said. "We are victims of persecution and have been made into scapegoats."

Vigil maintained it was untrue that the cartel would stop producing fentanyl because, "that is their big money maker." He also said that the rest of the Sinaloa Cartel "would never go along with" any move to stop the lucrative production.

"The Sinaloa Cartel strategy is to move away from plant-based drugs" like cocaine, marijuana and heroin, Vigil said. Giving up fentanyl — which could strengthen the rival Jalisco gang — "is going to give Jalisco the keys to basically overshadow them in terms of money."

In April, U.S. prosecutors unsealed sprawling indictments against Ovidio Guzmán and his brothers. They laid out in detail how following their father's extradition and eventual life sentence in the U.S., the brothers steered the cartel increasingly into synthetic drugs like methamphetamine and the powerful synthetic opioid fentanyl.

The indictment unsealed in Manhattan said their goal was to produce huge quantities of fentanyl and sell it at the lowest price. Fentanyl is so cheap to make that the cartel reaps immense profits even wholesaling the drug at 50 cents per pill, prosecutors said.

The Chapitos became known for grotesque violence that appeared to surpass any notions of restraint shown by earlier generations of cartel leaders.

Fentanyl has become a top priority in the bilateral security relationship. But López Obrador has described his country as a transit point for precursors coming from China and bound for the U.S., despite assertions by the U.S. government and his own military about vast fentanyl production in Mexico.

An estimated 109,680 overdose deaths occurred last year in the United States, according to numbers from the Centers for Disease Control and Prevention. About 75,000 of those were linked to fentanyl and other synthetic opioids.

U.S. prosecutors allege much of the production occurs in and around the state capital, Culiacan, where the Sinaloa cartel exerts near complete control.