Silicosis is a lung-scarring disease caused by inhaling silica dust, typically in occupations such as sandblasting. Silica sand is used to make glass and is a common ingredient in many industrial products.

Friday, the House committee announced it was now looking into the conduct of the lawyers involved in the cases as well.

The panel sent letters to 13 law firms and to authorities in six states where mass x-ray screenings were conducted to find victims to make the claims.

The silicosis lawsuits involved in the probe were first questioned last June when US Judge Janis Jack in Corpus Christi, Texas sent about 10,000 of the claims back to state courts, saying they had had been "manufactured for money."

The judge said many of the people diagnosed with silicosis had previously filed claims for asbestos-related disease, which medical experts said was highly unusual.

The House committee followed up with an investigation, but has not publicised its findings so far. A committee spokesman said Friday it was too soon to say if lawmakers would hold hearings or propose legislation.

The letters sent by the panel to the law firms questioned if a doctor-patient relationship existed with the mass screenings, noting some used mobile x-ray equipment in the back of a truck to find people to file the lawsuits. "It is hard to imagine circumstances where leaving such tasks to lawyers could be considered acceptable medical practice or serving the interest of public health," they said.

The House panel gave the law firms until March 3 to respond with information about their financial arrangements with doctors and medical screening companies.