Look out, e-tail scammers — the U.S. Senate's coming after you.

The Senate Commerce, Science, and Transportation Committee has launched a probe into deceptive online marketing practices that billed thousands of dollars in "mystery charges" to unsuspecting credit-card users, committee chairman Sen. Jay Rockefeller, D-W.Va., said Thursday.

"Thousands of American consumers have been complaining about these deceptive practices and asking for answers — and rightly so," Sen. Rockefeller said in a statement. "There could be many more affected by these hidden mystery charges — we must do all we can to protect the hard working Americans relying on us to look out for their wallets and well-being."

The scam stems from "a group of marketing companies that acquire consumers' billing information through agreements with popular online retail sites," according to the Senate committee.

Users of travel retailer Orbitz.com or movie-ticket seller Fandango.com, to use two examples cited, would see a pop-up window appear after they'd made a purchase. Other online retailers named were FTD, Petco and Priceline.com.

The pop-up would offer "cash back rewards" if the customer enrolled in an "online membership service" — which appears to be from the retailer in question, but is actually from a concealed third-party company.

After a trial period, often 30 days, the third-party company begins charging a monthly fee ranging from $9 to $12. That detail is buried in the fine print, and consumers often have no idea what the charges are for, or whom they're from, when they show up on credit-card statements.

Rockefeller sent letters Wednesday to Vertrue, Inc., and Webloyalty.com, Inc., two of the third-party billing companies involved, demanding more information about their practices. Both companies are based in Norwalk, Conn.

• Click here for the press release from the Commerce committee, which includes links to Sen. Rockefeller's letters.