Because unlicensed dealers are not required to inspect a buyer's driver's license or other identification, they may not even know that the purchaser is under the age of 18, the Administration said.

A ballot initiative in Colorado that would require background checks for those who buy guns from unlicensed dealers has such strong support that at least one Democrat is hoping to ride its coattails to victory.

After 4,000 inspections, said Jane Hoffman, the commissioner, the department found 28 unlicensed dealers in the city.

Then, report Mr. Anti-gun to the feds as an unlicensed dealer.

"It's cash and a handshake," Bob Glass, owner of Paladin Arms in Boulder, said of sales by unlicensed dealers at gun shows.

Ms. Anderson was not prosecuted because at the time no law prevented her from buying the "long guns" from an unlicensed dealer.

The gunman, Ben Smith, later bought a gun from an unlicensed dealer.

Mr. Clinton lent support to a proposed ballot measure in Colorado that would require unlicensed dealers at gun shows to run background checks of gun buyers within three days.

But the background checks are not required when unlicensed dealers sell at gun shows.

In California, the state legislature has approved a bill designed to help close off the gun supply to the illegal market run by unlicensed dealers on the streets by limiting handgun purchases to one a month.