In the wake of the shootings in Tucson, the familiar questions inevitably resurfaced: Are communities where more people carry guns safer or less safe? Does the availability of high-capacity magazines increase deaths? Do more rigorous background checks make a difference?

The reality is that even these and other basic questions cannot be fully answered, because not enough research has been done. And there is a reason for that. Scientists in the field and former officials with the government agency that used to finance the great bulk of this research say the influence of the National Rife Association has all but choked off money for such work.

“We’ve been stopped from answering the basic questions,” said Mark Rosenberg, former director of the National Center for Injury Control and Prevention, part of the federal Centers for Disease Control and Prevention, which was for about a decade the leading source of financing for firearms research.

Chris Cox, the N.R.A.’s chief lobbyist, said his group had not tried to squelch genuine scientific inquiries, just politically slanted ones.