WASHINGTON — Federal prosecutors have filed multiple charges against a 23-year-old South Korean man accused of running what they call the world's "largest dark web child porn marketplace."

The now-shuttered English-language site, called "Welcome to Video," contained more than 200,000 unique videos or almost 8 terabytes of data showing sex acts involving children, toddlers and infants, according to the 18-page criminal indictment unsealed here Wednesday, and processed 7,300 Bitcoin transactions worth more than $730,000.

According to prosecutors, the vast online store was run by Jong Woo Son, a South Korean citizen currently serving an 18-month prison sentence in his home country after his conviction on charges related to child pornography. The site operated from June 2015 until it was seized and shut down by U.S. authorities in March 2018.

At a press conference Wednesday morning, U.S. officials said 337 suspected users of the site had been arrested worldwide to date.

U.S. Attorney Jessie Liu and other officials describe their investigation of a vast dark web child porn marketplace during a press conference in Washington, on Oct. 16, 2019. Cyrus Farivar / NBC News

"You may try to hide behind technology," U.S. Attorney for the District of Columbia Jessie Liu said, "but we will find you and arrest you and prosecute you."

Almost half of the videos seized were not previously known to the National Center for Missing and Exploited Children, Deputy Assistant Attorney General Richard Downing said. "There are many children depicted in these videos that have not been identified."

"The scale of this crime is eye-popping and sickening," John D. Fort, chief of IRS criminal investigations, said.

The site allegedly rewarded its members with "points" in relation to the amount of illegal material they uploaded, authorities announced Wednesday.

One user of the site, a former federal U.S. law enforcement agent, even downloaded more than 50 hours worth of videos.

Son was indicted on federal child porn charges in August 2018, which remained sealed until Wednesday.

In addition to Son, more than 300 other suspects have been arrested in South Korea as of Wednesday, while still more suspects were identified in other countries, including the United Kingdom and the United States, including a Washington, D.C., man who was caught with the equivalent of 50 years worth of video footage he had downloaded.

The website ran solely on the dark web, a section of the internet that can only be accessed via a Tor browser, which is designed to protect users' tracks online and obscure digital footprints. Users could purchase videos using cryptocurrency and an annual membership was priced at 0.03 bitcoins (at current exchange rates, around $300).

This warning was posted on "Welcome to Video" after it was seized by U.S. authorities. Department of Justice

Members earned points by uploading new and popular videos to the site or by inviting new members. The site sought only child sexual abuse imagery, according to prosecutors — its landing page stated in red bold type: "Do not upload adult porn." Prosecutors have also filed a related civil forfeiture case, seeking to seize Bitcoin assets held across 24 different accounts.

A notable moment in the investigation came in September 2017 when an Internal Revenue Service agent who was assisting with the probe noticed that the site's administrator had misconfigured his site, which revealed his South Korean IP address, and then ultimately revealed the identity of its owner. During the criminal probe, authorities conducted controlled buys of child porn videos from the site and eventually worked closely with the U.S. attorney's office in D.C. on the investigation.

Ultimately, the site was seized in March 2018, when Son was arrested by South Korean authorities. The Korean National Police Agency (KNPA) announced the arrest of a "Mr. A" from Danjin, roughly two hours southwest of Seoul, on May 2, 2018. They later confirmed to NBC News that this was Jong Woo Son.

Liu said Wednesday she could not comment on "what may be happening with respect to extradition."

Five U.S. defendants identified

NBC News has been able to identify five defendants in the U.S. who have faced criminal charges related to the site.

One user, Nicholas Stengel, of Washington, D.C., pleaded guilty to child porn charges late last year and was sentenced to 15 years in prison: prosecutors said that he alone downloaded more than 50 years worth of abuse videos.

In April 2019, Mark Rohrer, a man from West Hartford, Connecticut, was sentenced to five years in prison after pleading guilty in September 2018 to similar charges.

Other defendants include Richard Gratkowski, a former Homeland Security Investigations agent, who pleaded guilty and was sentenced to 70 months in prison back in May 2019. According to the FBI, Gratkowski used Coinbase, a well-known online wallet company, with his own government passport as identification.

Also in May 2019, across the country in Rhode Island, an Army veteran named Stephen P. Langlois was sentenced to 42 months in prison for downloading 114 videos from the site.

A Texas defendant, Michael Ezeagbor, pleaded guilty earlier this year to downloading 42 videos and uploading 10 videos. His sentencing is scheduled for Friday. The government has asked for at least seven years and 10 years of supervised release.

When they announced the arrest of "Mr. A" in 2018, the South Korean police also said they had arrested a total of 156 South Koreans for either uploading or downloading child porn materials, which was unusual given that the site operated entirely in English.

"Most of the users were in their 20s, unmarried and white-collar office workers and first-time offenders, although some were ex-convicts of sexual crimes, including juvenile sex offenders. One possessed as many as 48,634 child porn [files]," the KNPA said.

Paul Jenkins, head of the Americas region for the U.K.'s National Crime Agency, said at the Wednesday press conference that 18 investigations of alleged site users had yielded seven convictions, with one defendant sentenced to 22 years.

The case, Jenkins said, demonstrates the "increase in the scale, severity and complexity of child sexual abuse offending."

Andrew Blankstein reported from Los Angeles. Stella Kim contributed reporting from Seoul.