WASHINGTON — Federal regulators have been investigating and are close to filing lawsuits against companies believed to be behind a national wave of spam "robocalls" that warn people that their auto warranties are about to expire and they need to sign up for new service plans.

The Federal Trade Commission has inquiries under way into several companies involved in the deceptive calls, and "law enforcement action in this area can be expected imminently," FTC Chairman Jon Leibowitz said in a letter to Sen. Charles Schumer, D-N.Y.

The FTC also is providing an electronic link on its home page for consumers to file complaints, Liebowitz said.

Schumer had asked for an investigation by the agency into what he calls a scam of "robo-dialer harassment." The computerized calls can eat up a consumer's cell phone minutes, possibly jacking up phone charges, he said.

The message "Your Car Warranty Has Expired" already has brought some 300,000 complaints nationwide, according to Schumer. The calls come even if the consumer has signed up for the national Do Not Call registry, which is operated by the FTC.