At least 95% of cryptocurrency crimes investigated by law enforcement involve bitcoin (BTC,) the co-founder and COO of Chainalysis told Fortune on April 24.

Jonathan Levin, whose company offers investigation software for law enforcement to pursue bad actors, said BTC is “by far the favorite” for hackers and criminals.

He revealed that law enforcement needs to take more sophisticated approaches to tackle darknets — and warned that the crypto industry was starting to see the beginnings of terrorism financing.

Levin said the records left behind by crypto transactions has led to many arrests, as officials in the United States tackle the deadly opioid crisis and try to stem the flow of illegal drugs into the country, often from China. He told Fortune:

“What we’ve seen is that there is the ability to tie some of those cryptocurrency transactions either to the pharmacies in China or actually to the services that people are using to distribute fentanyl.”

According to Levin, the transparency of cryptocurrencies is helping law enforcement to build cases against suspects quicker than in traditional finance, namely because investigators no longer need to rely on obtaining records from foreign banks.

Even though bitcoin is overwhelmingly used by crypto criminals, Levin noted that Chainalysis has launched real-time transaction monitoring for 10 cryptocurrencies, as well as a basket of stablecoins, because law enforcement agencies are trying to investigate hacks on crypto exchanges where other tokens are stolen.

Levin also said he has been investigating the case of QuadrigaCX, the major Canadian crypto exchange that was officially declared bankrupt earlier this month. The company lost access to cold wallets and corresponding keys following the CEO’s death in December last year, and reportedly owes more than $195 million to 115,000 customers.

In the interview, the Chainalysis executive suggested Quadriga “was operating on fractional reserve and in deep trouble”:

“We were looking at the bitcoin holdings and also tether holdings of Quadriga and what we found very quickly was that Quadriga as an exchange actually didn’t have those customer funds that were reported in the media to be lost. Those funds actually never existed.”

Levin also repeated claims made in a Chainalysis report earlier this year that two hacker groups have been responsible for stealing $1 billion in cryptocurrency, representing the majority of the funds lost in scams.