THE SHOCK waves continue to reverberate from Justice Department Inspector General Michael Horowitz’s Dec. 9 report on major flaws in the federal government’s secret surveillance of former Trump campaign official Carter Page. Mr. Horowitz found that FBI officials repeatedly misled or misinformed judges of the Foreign Intelligence Surveillance Court (FISC) to obtain its permission for the surveillance of Mr. Page. Now the FISC has revealed that the Justice Department itself has acknowledged it had no legitimate legal basis for at least the last two of four surveillance warrants it obtained against Mr. Page in 2016 and 2017. That admission came in a previously classified letter the department sent the FISC on the day Mr. Horowitz’s report came out. Its publication should add to the urgency with which the executive branch, Congress and the FISC address the problems Mr. Horowitz identified.

Credit goes to the newly appointed head of FISC, U.S. District Judge James E. Boasberg, for making sure that this crucial information entered the public domain; he included it in an order to the government seeking guarantees that improperly gathered information about Mr. Page will not be disclosed, which he then declassified. Judge Boasberg correctly understands that public trust is essential to the legitimacy of the largely — and necessarily — secret FISC process. Mr. Boasberg further advanced that trust by appointing former Justice Department national security official David Kris as an adviser to the FISC on reforming FBI procedures.

Attacked by President Trump and his supporters for his previous defense of the FBI in the Carter Page matter, Mr. Kris has actually demonstrated an open mind and a willingness to revise his views in light of the Horowitz report. His first report to the FISC, on Jan. 15, called the FBI’s reform proposals “insufficient” in light of the way it “profoundly failed” in the Page case. He recommended that the government subject a certain number of randomly selected FISC warrants to “in-depth” retrospective checks for accuracy, to deter any repeat of the errors of omission that plagued the Page warrants.

Congress, too, has a responsibility to weigh in on this critical civil liberties issue — and the impending expiration of key provisions of the Foreign Intelligence Surveillance Act (FISA) in March provides an opportunity to do so. If the Page case illustrates anything, it is the fact that no U.S. citizen, of any political party, is immune to unwarranted surveillance by a misguided or overzealous FBI. Reform, therefore, should be a bipartisan cause. Republican Sen. Steve Daines (Mont.) and Democrat Sen. Ron Wyden (Ore.) have proposed a bill that would inject a greater degree of transparency and rigor into the FISC’s internal processes. Adopted in 1978 as a response to the intelligence agency abuses of the 1960s and 1970s, FISA needs an update in response to the abuses Mr. Horowitz laid bare.

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