Along with scores of other immigrants, Mal Soon Jin showed up early yesterday morning to the federal courthouse in Brooklyn. After 20 years in America, Mrs. Jin was eager to take the oath of citizenship. Eight hours after arriving, she was pleading for her freedom and was farther away than ever from becoming an American.

One of the best days of Mrs. Jin’s life suddenly became one of her worst when she placed her handbag — allegedly containing makeup, a notebook, keys, and a five-shot Derringer — on the X-ray machine that screens the items of each visitor to U.S. District Court in Brooklyn.

Having studied for citizenship, Mrs. Jin surely knew of the Bill of Rights, and is perhaps even familiar with the Second Amendment. However, federal law prohibits people from carrying guns in the courthouse — as three signs at the entrance to the courthouse point out.

It is a crime usually punished by between six and 16 months in prison, Mrs. Jin’s attorney, Peter Kirchheimer, said at an afternoon arraignment that occurred across the hall from where Ms. Jin had been scheduled to become a citizen.

The small gun recovered by authorities had a chrome and wood finish and was unloaded, a law enforcement source said. It appeared bright blue on the monitor that a court security officer watched closely. When confronted, Mrs. Jin denied owning the gun and suggested that one of the six officers manning the entrance had placed it in her bag, the source said.

The chief deputy U.S. marshal of the courthouse, Timothy Hogan, said he did not know of a similar instance in which a person had tried to bring in a handgun.

Appearing at her arraignment were Mrs. Jin’s husband and brother-in-law. The day was supposed to be a happy one for the family, the brother-in-law, Francis Jin, said.

“She’s been studying for years,” he said of Mrs. Jin’s efforts to become naturalized. “Everyone wants to become a citizen.”

Mrs. Jin, 49, had worked at the family’s fruit store in Brooklyn, Mr. Jin said. She came to America from Korea in 1986, he said. Mrs. Jin was tearful and seemed stunned to silence yesterday as she filled out paperwork relating to her release. She declined to be interviewed.

In court, Mr. Kirchheimer told the magistrate-judge, Cheryl Pollack, that he believed a jury could be convinced of Mrs. Jin’s innocence.

As long as Mrs. Jin was not aware of the gun in her handbag, Mr. Kirchheimer said, she was innocent of the federal charge, which requires that the person “knowingly” bring the gun to court.

Why, Mr. Kirchheimer asked, would Mrs. Jin, who had watched dozens of other immigrants ahead of her in line go through the screening, put her handbag through the X-ray machine had she been aware of the gun?

“If she wins the case,” Mr. Kirchheimer said, “she still gets naturalized.”