Leading crypto critic U.S. Sen. Elizabeth Warren (D-Mass.) has asked U.S. Attorney General Merrick Garland what tools the federal government needs to crack down on the use of digital assets to perpetrate child sexual exploitation, according to a letter she sent to the top U.S. law-enforcement official.

Warren and Sen. Bill Cassidy (R-La.) flagged cryptocurrencies as a significant tool supporting child sexual abuse materials in a letter the senators sent this week to Garland and Alejandro Mayorkas, the chief of the U.S. Department of Homeland Security.

Cryptocurrency has become "the payment of choice for perpetrators of child sexual abuse and exploitation," the lawmakers noted, citing a February FinCEN Trend Analysis from transactions in 2020 and 2021 and a Chainalysis report from this past January. "Existing anti-money laundering rules and law enforcement methods face challenges in effectively detecting and preventing these crimes – and we seek to ensure that Congress and the administration are doing their part to address these challenges."

The letter asked the agencies to detail the "additional tools and resources" they need to deal with the problem.

The use of digital tokens to fund the abuse of children rose to prominence years ago, such as in the 2019 bust of the Welcome to Video pornography site run by a South Korean national and, a year later, when authorities pursued a Dutch national who ran a rape and child porn site on the darkweb and made $1.6 million in bitcoin (BTC).

Crypto became a favorite form of payment due to perceptions that it offered anonymity for the transactions, though analytic tools and law-enforcement strategies have since cast some doubt on that. Much of the data on crypto use cited by the lawmakers is several years old. Still, the Chainalysis review earlier this year noted it as "a growing problem."

Read More: Bitcoin Favored in Human Trafficking, Child Exploitation: FinCEN Report