U.S. Attorney General Jeffrey Sessions is concerned about the use of bitcoin and other cryptocurrencies in illicit transactions.

During his testimony to the Senate Judiciary Committee on Wednesday, Sessions – a former U.S. senator himself – highlighted the use of bitcoin in response to a question by Senator Dianne Feinstein (D-CA) about dark markets, or online marketplaces

Sessions told the committee:

“[Dark web users] use bitcoins and other untraceable financial capabilities and it is a big problem.”

In his remarks, Sessions referred to the closure of two dark markets, including previous ecosystem leader AlphaBay, following a crackdown by US authorities. Sessions claimed that AlphaBay’s client base constituted “240,000 accounts where individuals were selling for the most part illegal substances and guns, including fentanyl.”

Law enforcement officials have long expressed concern about the use of cryptocurrencies by dark markets, arguing that they obscure the paper trail that investigators might follow. Agencies like the Federal Bureau of Investigation have targeted this use case when seeking funds to bolster their cybercrime efforts, a trend that arguably began in the wake of the closure of the Silk Road, once the world’s top dark market.

Sessions’ appearance yesterday also led to indications that Congress may take up specific measures related to dark markets.

In her remarks, Sen. Feinstein stated that she would like to work with Sessions and the Justice Department on issues surrounding the dark web, potentially through legislation or alternative actions.

Image via mark reinstein / Shutterstock.com.