Oakland Mayor Libby Schaaf said Thursday she is preparing for the worst-case scenario in the Department of Justice’s reported investigation into her February warning to undocumented immigrants about an impending federal sweep.

Schaaf said she expects investigators to use “all the means they have” to track down how she found out about the four-day Northern California Immigration and Customs Enforcement operation ahead of time.

“I am assuming the worst,” she told reporters Thursday. “I am preparing for the worst.”

Her remarks came after CBS News aired an interview with former ICE spokesman James Schwab that was interrupted when investigators with the Department of Homeland Security knocked on his door and asked to speak with him.

“They just said that they wanted to talk to me about the leak with the Oakland mayor,” Schwab told CBS News after the surprise house call Wednesday. Schwab said he had never contacted Schaaf, whose warning, he added, put federal officers’ safety at risk.

The night of Feb. 24, Schaaf released a statement saying that ICE was planning arrests of immigrants across the region as early as the following day. She said her information came from confidential “credible sources.” Her warning included information on how to obtain legal representation and other resources.

Schwab quit the San Francisco ICE office in March, saying he did not want to perpetuate false statements from Attorney General Jeff Sessions and other administration officials, who said roughly 800 immigrants eluded arrest because of Schaaf’s public warning. Schwab said the figure was exaggerated.

The Justice Department opened a review of Schaaf’s actions, White House press secretary Sarah Huckabee Sanders has said. The department did not immediately respond to a request for comment Thursday on the status of that review.

Schaaf told The Chronicle that she had not received any subpoenas or requests for information from any federal agency. She also said she had no communication with Schwab.

“I will not back down,” Schaaf said. “I’m very clear about what the values of the people of Oakland are, and I believe it is my job to continue to stand up for those values.”

Former U.S. Attorney Melinda Haag has volunteered to represent the mayor, pro bono, should the administration press charges against her. Legal experts have said that an obstruction-of-justice case would be difficult to make because Schaaf’s warning was so broad and did not encourage anyone to commit illegal acts.

Schaaf, who watched the Schwab interview, called investigators knocking on his door “the sound of tyranny.”

“The idea that the Trump administration is continuing to intimidate and vilify good public servants — someone who has stood up for the simple idea that government should not lie — just offends everything I believe about government and democracy,” she said.

Kimberly Veklerov is a San Francisco Chronicle staff writer. Email: kveklerov@sfchronicle.com Twitter: @kveklerov