Since the attempted bombing of a U.S. airliner on Christmas Day, former Homeland Security secretary Michael Chertoff has given dozens of media interviews touting the need for the federal government to buy more full-body scanners for airports.

What he has made little mention of is that the Chertoff Group, his security consulting agency, includes a client that manufactures the machines. The relationship drew attention after Chertoff disclosed it on a CNN program Wednesday, in response to a question.

An airport passengers' rights group on Thursday criticized Chertoff, who left office less than a year ago, for using his former government credentials to advocate for a product that benefits his clients.

"Mr. Chertoff should not be allowed to abuse the trust the public has placed in him as a former public servant to privately gain from the sale of full-body scanners under the pretense that the scanners would have detected this particular type of explosive," said Kate Hanni, founder of FlyersRights.org, which opposes the use of the scanners.

Chertoff's advocacy for the technology dates back to his time in the Bush administration. In 2005, Homeland Security ordered the government's first batch of the scanners -- five from California-based Rapiscan Systems.

Today, 40 body scanners are in use at 19 U.S. airports. The number is expected to skyrocket at least in part because of the Christmas Day incident. The Transportation Security Administration this week said it will order 300 more machines.

In the summer, TSA purchased 150 machines from Rapiscan with $25 million in American Recovery and Reinvestment Act funds. Rapiscan was the only company that qualified for the contract because it had developed technology that performs the screening using a less-graphic body imaging system, which is also less controversial. (Since then, another company, L-3 Communications, has qualified for future contracts, but no new contracts have been awarded.)

Over the past week, Chertoff has repeatedly talked about the need for expanding the use of the technology in airports, saying it could detect bombs like the one federal authorities say Umar Farouk Abdulmutallab, a 23-year-old Nigerian, carried onto the Detroit-bound aircraft.

"We could deploy the scanning machines that we currently are beginning to deploy in the U.S. that will give us the ability to see what someone has concealed underneath their clothing," Chertoff said Wednesday in an interview on CNN. The incident on the Detroit-bound plane provided "a very vivid lesson in the value of that machinery," he said.

Staff researcher Julie Tate contributed to this report.