The Justice Department has arrested one of the biggest sellers of drugs on the dark web and moved to bar two Ohio doctors from prescribing medications, part of a sprawling effort to prosecute drug cases amid the nation’s opioid epidemic.

It is one of several arrests, charges and guilty pleas involving illicit and prescription drugs announced in Cleveland on Wednesday, as Attorney General Jeff Sessions is moving aggressively against those who distribute illicit drugs and illegally prescribe legal ones.

“Today’s announcements are a warning to every trafficker, every crooked doctor or pharmacist, and every drug company, every chairman and foreign national and company that puts greed before the lives and health of the American people: this Justice Department will use civil and criminal penalties alike, and we will find you, put you in jail, or make you pay,” Sessions said.

Federal prosecutors allege that Matthew and Holly Roberts of San Antonio were two of the biggest drug dealers on the dark web, completing nearly 3,000 verified transactions on various underground marketplaces between 2011 and 2018 — including the largest number of verified fentanyl transactions on the dark web. Prosecutors said they operated under the name “MH4Life.” They could not be reached for comment Wednesday.

In addition to the powerful synthetic opioid fentanyl, which has caused the number of overdose deaths nationwide to skyrocket, authorities allege the couple possessed and distributed fentanyl analogues, heroin, cocaine, methamphetamine, Xanax and other drugs. They allegedly bought postage with cryptocurrency to conceal their intent and used glow bracelets and other items to hide that they were mailing drugs, authorities said. Customers used digital currency to buy the drugs and conceal the deals, prosecutors said.

The charges are part of “Operation Darkness Falls,” which targeted those who sell fentanyl and other drugs on the dark web. Federal prosecutors charged five other people in connection with the operation, including “DF44,” the third-largest fentanyl vendor in North America, who was arrested in Canada and later committed suicide; and “Dark King 22,” an Ohio man who allegedly ran an Internet fentanyl business from an apartment where children lived.

“These cases demonstrate that those who think they are hiding behind a cloak of anonymity on the dark net will be uncovered and brought to justice for selling the drugs killing our friends and neighbors,” said Justin E. Herdman, the U.S. attorney for the Northern District of Ohio, who appeared with Sessions in Cleveland.

Prosecutors also charged two Chinese men with a wide-ranging conspiracy that involved importing drugs and money laundering. Fujing Zheng, known as Gordon Jin, and his father, Guanghua Zheng, allegedly manufactured and shipped fentanyl analogues and 250 other drugs to at least 25 countries and throughout the United States. Prosecutors said the drugs led to the overdose deaths of a man and woman in Ohio in 2015.

Authorities said the men were part of the Zheng drug trafficking organization, which started up in 2008. Authorities said it boasted of its ability to manufacture custom drugs and skirt enforcement in the United States, Russia and Europe, and has sent millions of doses of fentanyl and its analogues around the world.

Sessions also filed temporary restraining orders against two Ohio doctors who were found to overprescribe drugs, the first such federal action against medical professionals.

Authorities said the moves against Michael Tricaso of Akron and Gregory J. Gerber of Sandusky will prohibit them from writing prescriptions.

“The physicians in this investigation were nothing short of automatic prescription machines to anyone who solicited,” Timothy Plancon, special agent in charge of the Detroit field division, said in a statement. He said their “reckless actions and corruption” have had a major impact on the opioid crisis that is “plaguing America.”

Officials said Triasco met a confidential source in a hotel parking lot and offered to provide Percocet without a prescription; he later sold pills to the source and wrote him a prescription. Writing a prescription for 20 pills, Triasco allegedly texted, would be an “under the radar amount and won’t be a red flag.”

Authorities said Gerber received $175,000 from Insys Therapeutics, whose founder was charged last year with conspiracy to distribute fentanyl. Prosecutors said the payments to Gerber violated laws against kickbacks. Garber also allegedly treated an undercover agent, who was able to receive powerful prescription drugs including Xanax and OxyContin with just a cursory physical examination.

Triasco and Gerber could not be reached for comment.

The actions come as part of a concerted effort by the Justice Department to ratchet up prosecutions as the number of opioid overdose deaths continue to rise. More than 72,000 people died of drug overdoses last year, according to preliminary 2017 figures from the Centers for Disease Control and Prevention. That is a 9.5 percent increase from 2016, a rise driven largely by deaths from fentanyl and carfentanil, an even stronger opioid typically used as a large-animal tranquilizer.