WASHINGTON, July 11 — Undercover Congressional investigators set up a bogus company and obtained a license from the Nuclear Regulatory Commission in March that would have allowed them to buy the radioactive materials needed for a so-called dirty bomb.

The investigators, from the Government Accountability Office, demonstrated once again that the security measures put in place since the 2001 terrorist attacks to prevent radioactive materials from getting into the wrong hands are insufficient, according to a G.A.O. report, which is scheduled to be released at a Senate hearing Thursday.

“Given that terrorists have expressed an interest in obtaining nuclear material, the Congress and the American people expect licensing programs for these materials to be secure,” said Gregory D. Kutz, an investigator at the accountability office, in testimony prepared for the hearing.

The bomb the investigators could have built would not have caused widespread damage or even high- level contamination. But it still could have had serious consequences, particularly economic ones, in any city where it was set off.