During a hearing of the Senate Judiciary Committee, the California Democrat produced a recent letter from a top federal immigration official in San Diego praising Lam's record in combating immigrant smuggling.

The three-page letter, dated Feb. 15, casts doubt, Feinstein said, on testimony by Kyle Sampson, who resigned this month as chief of staff to Attorney General Alberto Gonzales amid a growing controversy over the dismissals of Lam and seven other U.S. attorneys.

“It is a real surprise to me that you would say here that the reason for her dismissal was immigration cases,” Feinstein told Sampson during the hearing.

The letter, by Adele Fasano, director of field operations for the San Diego office of U.S. Customs and Border Protection, said that Lam's efforts last year contributed “to the reduction by at least 50 percent (in) the number of smuggled aliens encountered at the San Diego ports of entry.”

In an interview, the senator added, “I don't think the director of field operations would write a letter commending (Lam) unless she really believed this woman was doing an excellent job.”

Fasano was unavailable for comment about the letter.

In his testimony, he insisted that her sacking had nothing to do with her office's successful prosecution of former Rep. Randy “Duke” Cunningham, R-Rancho Santa Fe, and the grand jury indictments of Kyle “Dusty” Foggo, the former third-ranking official in the CIA, and defense contractor Brent Wilkes of Poway. The indictments came two days before Lam left office Feb. 15, though she had announced weeks earlier that she would be leaving.

“There was never any connection in my mind between asking Carol Lam to resign and the public corruption case that her office was working on,” Sampson said.

According to e-mails and other documents recently released by the department, Lam notified her superiors May 10 of her intention to issue search warrants in connection with her investigation of Foggo and Wilkes. The next day, Sampson e-mailed a White House official about “a real problem” with Lam and arguing that she should be replaced at the expiration of her term Nov. 18.

Earlier on the day of the Sampson e-mail, the Los Angeles Times reported that the U.S. attorney in Los Angeles had opened an investigation of Rep. Jerry Lewis, R-Redlands, that was connected to the Cunningham case.

Asked about that e-mail by Sen. Arlen Specter, R-Pa., Sampson said that the problem with Lam, in his mind, was “immigration enforcement.”

Later, in response to questions from Sen. Orrin Hatch, R-Utah, Sampson described Lam as “a good person and a very skilled lawyer.”

But he said her name was initially included on a list of U.S. attorneys earmarked for dismissal “due to her office's failure to embrace the president's anti-gun violence initiative, Project Safe Neighborhoods.”

In the e-mails, Justice Department officials criticized Lam for not enough immigration prosecutions.

That echoed concerns from Republican congressmen, including Rep. Darrell Issa of Vista. Feinstein at one point last year inquired about the immigration prosecutions, but later said she was satisfied after hearing from Justice Department officials who defended Lam on the issue.

After her dismissal, Lam said she was never told about concern within the department about her immigration record. She also said her practice was to focus on fewer, bigger cases that produced longer prison sentences.