WASHINGTON — The Bureau of Alcohol, Tobacco, Firearms and Explosives is relaxing restrictions on the sale of guns to noncitizens because Justice Department lawyers have concluded that the rules had no legal basis, officials said on Thursday.

In a letter to firearms dealers on Thursday, the bureau said it would soon drop a regulation that bars the sale of guns to noncitizens until they can document that they have lived in a state for at least 90 days, such as by producing three months of utility bills in their name at a local address.

While citizens, too, must generally be residents of a state in order to buy weapons there, the 90-day rule does not apply to them. The letter said the Justice Department had concluded that the Gun Control Act does not empower the A.T.F. to impose a stricter requirement on noncitizen gun buyers.

“Once the regulations have been revised, both U.S. citizens and aliens legally present in the U.S. will be subject to the same requirements for state residency and proof of residency,” the A.T.F. said in the letter.