U.S. authorities have opened a new front in their investigation into bitcoin exchanges and other businesses that deal in the online currency, examining possible ties between the firms and the online drug bazaar Silk Road, according to people familiar with the matter.

The new focus of the investigation is the latest indication of how the shuttered online drug market has become linked to bitcoin, tainting the currency with associations of shady activity as its advocates stress to regulators its legitimacy. It comes after prosecutors in January criminally charged the owner of one bitcoin exchange over alleged ties to Silk Road.

One of the people familiar with the matter said the investigation was at an early stage, and conclusions hadn't been reached as to whether the exchanges were connected with Silk Road.

Several bitcoin exchanges received subpoenas from Manhattan federal prosecutors this winter, including Tokyo-based Mt. Gox, once the world's most popular bitcoin exchange, according to people familiar with the matter. It wasn't clear which other bitcoin businesses have received subpoenas.

The subpoenas to Mt. Gox demanded customer-transaction logs and materials related to solicitation of investors. But prosecutors and Federal Bureau of Investigation agents also are examining whether the exchange and others like it may have processed transactions connected to Silk Road, the people said. Attorneys for Mt. Gox didn't respond to requests for comment.