Nouriel Roubini, the former Clinton administration economist nicknamed "Dr. Doom" due to his prediction of the 2008 financial collapse, resumed his criticism of cryptocurrencies on Twitter Tuesday.

Just prior to a Senate Committee hearing on cryptocurrencies, he wrote, "Cryptocrazies are also criminal Cyber-Terrorists," and alleged that his consulting firm, Roubini Global Economics, was targeted in a 2015 denial-of-service attack because he criticized bitcoin.

Roubini further claimed that he had received death threats as a result of his views. Roubini said in the tweet that he anticipated another attack, and had received recent threats..

These alleged threats did not force the economist to back down from his stance on bitcoin, however. In another tweet, the New York University professor praised Agustin Carstens, the head of the Bank for International Settlements (BIS), for labeling bitcoin as a "Ponzi scheme" in a speech Tuesday.

In other tweets, Roubini posted links to several articles claiming a connection between bitcoin and criminal activity. At the same time, he noted bitcoin's recent price decline and the overall cryptocurrency market correction, saying that bitcoin is starting "to look like a dinosaur on the way to extinction," and suggested that its value will remain volatile.

As previously reported by CoinDesk, Roubini has been a consistent and vocal critic of bitcoin and cryptocurrencies more generally.

Last month, he condemned the notion that cryptocurrencies could replace fiat currencies as "utterly idiotic." In the past, he has also dismissed bitcoin as a bubble and a "Ponzi game."

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