Federal authorities announced Wednesday that in recent months, they have arrested, charged, and received guilty pleas from suspects in numerous new drug cases stemming from dark Web sites. Prosecutors collectively dubbed this project "Operation Darkness Falls."

The most prominent of these cases involves Matthew Roberts and Holly Roberts, a possibly married couple from San Antonio, Texas that jointly operated under the name "MH4Life."

According to federal prosecutors, the two suspects were "the most prolific dark net fentanyl vendor in the United States and the fourth most prolific in the world" at the time of their April 2018 arrest.

The Centers for Disease Control and Prevention said last year that fentanyl can be 50 to 100 times more potent than morphine, and it is often mixed with heroin or cocaine. The drug, which is legally prescribed for pain relief, can be highly addictive.

Prosecutors say that the Roberts duo had multiple accounts across several sites, including Dream Market, Silk Road, AlphaBay, Darknet Heroes League, Nucleus, and more. (Over a year has passed since AlphaBay, one of the world's largest online drug websites, was seized and shuttered.)

Under the Trump administration, the Department of Justice has gone hard after dealers on the dark Web : it indicted two Chinese nationals in October 2017.

Also on Wednesday, DOJ officials announced that two Chinese men—a father and son duo—were formally indicted in Ohio for manufacturing fentanyl analogues and hundreds of other drugs that were shipped to the United States.

"These traffickers allegedly manufactured—in China—more than 250 types of synthetic drugs, including synthetic opioids and deadly fentanyl analogues that they distributed in the United States and around the world," Attorney General Jeff Sessions said in a speech in Cleveland on Wednesday. "Using an elaborate system of shell companies, the Zheng organization allegedly shipped synthetic opioids to locations in more than 25 countries and 37 US states, including Ohio. The man who is charged with serving as the American reshipper for Zheng has already been arrested. He was caught while trying to flee the country and has pled guilty."