Two congressmen wrote to the Department of Interior this week to demand information on the progress of its investigation into whether more than 1,700 federally protected wild horses sold to a Colorado man may have been illegally sent to slaughter.

The investigation was triggered by a ProPublica story published in September showing that since 2009 the department had sold truckload after truckload of mustangs to a horse-slaughter proponent named Tom Davis who acknowledged shipping many of them out of state in violation of Colorado law. The fate of the horses is unknown.

Reps. Raul Grijalva, D-Ariz., and Ed Whitfield, R-Ky., have sent a letter to Interior Secretary Ken Salazar pressing him to explain what role the agency had in selling so many horses to a man the letter calls "a longtime kill buyer."

"As you are aware, the ProPublica revelations have provoked a substantial public outcry," the letter says. "Leading organizations and the public have stepped forward to express their concern with the idea that America's wild horses could be sold by their government to be slaughtered. ... We are troubled by your Department's lack of response to [their] legitimate concerns."

The pair of legislators also has begun circulating the letter in Congress, collecting signatures from other lawmakers, said Adam Sarvana, a spokesman for Grijalva, the top Democrat on the House Subcommittee on Public Lands and Environmental Regulations.

In the wake of the ProPublica report, the Bureau of Land Management announced it had opened an investigation into Davis' purchases. In October, the investigation was taken over by the Department of Interior’s Inspector General, who handles matters that could involve agency staffers. That investigation is ongoing.

Sarvana said Grijalva and Whitfield want answers before Salazar leaves his Cabinet position at the end of March. Their letter asks for a response within 10 days.