The Obama administration sought to intimidate witnesses into not testifying to Congress on Tuesday about whether ATF knowingly allowed weapons, including assault rifles, to be “walked” into Mexico, the chairman of a House committee investigating the program said in an interview Monday.

House Oversight and Government Reform Committee Chairman Darrell E. Issa, California Republican, said at least two scheduled witnesses expected to be asked about a controversial weapons investigation known as “Fast and Furious”received warning letters from the Bureau of Alcohol, Tobacco, Firearms and Explosives to limit their testimony.