The Justice Department has opened a probe into whether the credit card processing company Visa is engaging in anti-competitive behavior in the debit-card market, a source familiar with the matter told Reuters on Friday.

More specifically, the DOJ's anti-trust division wants to know if Visa is blocking merchants' ability to rout debit-card transactions, which are usually less expensive, over card networks, according to the Wall Street Journal.

Visa declined a Reuters request for comment and the DOJ did not immediately respond.

The speculation, however, was enough to knock Visa shares lower... with the stock at one-point losing as much $25 billion in market value in Friday trading.

Shares of Visa are on the decline so far this year.