Attorney General Jeff Sessions said Friday that the Justice Department has more than tripled the number leak investigations compared to the number that were ongoing at the end of the last administration, offering the first public confirmation of the breadth of the department’s efforts to crack down on unauthorized disclosures of sensitive information.

Sessions made the announcement at a long anticipated news conference with his deputy, Rod Rosenstein, as well as Director of National Intelligence Daniel Coats and National Counterintelligence and Security Center Director William Evanina. No representative of FBI, which typically investigates leak cases, appeared on stage.

Sessions said in the first six months of this year, the Department of Justice had received nearly as many criminal referrals involving unauthorized disclosures of classified information than it had received in the past three years combined. Though he did not say if it resulted in a criminal referral, Sessions cited in particular a recent disclosure to The Washington Post of transcripts of President Donald Trump’s conversations with Mexican President Enrique Pena Nieto and another with Australian Prime Minister Malcolm Turnbull.

Sessions said prosecutors had charged four people with leaking or “concealing contacts with federal officers.” It was not immediately clear to which prosecutions he was referring. Only one leak prosecution had been previously known.

“This culture of leaking must stop,” Sessions said.

Trump has complained vociferously about unauthorized disclosures of information — casting the issue as more worthy of attention than the investigation into whether his campaign coordinated with the Kremlin to influence the 2016 election.

Sessions, too, has said previously illegal leaks are “extraordinarily damaging to the United States’ security” and confirmed that such disclosures were “already resulting in investigations.” His work on the matter, though, has apparently not been to the president’s satisfaction. Last week, Trump wrote on Twitter that his attorney general had taken a “VERY weak position” on “Intel leakers.”

Leak cases are difficult to prove and prosecute, and they almost always come with political controversy — especially when the leaks involve providing information to reporters that is arguably in the public interest.

Attorney General Eric Holder issued new guidelines in 2015 to the department’s policy on obtaining information from members of the news media, after his Justice Department came under fire for the tactics prosecutors used in bringing such cases. Sessions said Friday he is reviewing the Justice Department’s policy on issuing subpoenas to reporters.

The Obama administration had taken an especially aggressive stance on leaks. Prosecutors in the Obama era brought nine such cases, more than during all previous administration combined, and in the process, called a reporter a criminal “co-conspirator,” and secretly went after reporters’ phone records in a bid to identify reporters’ sources. Prosecutors in the Obama administration also sought to compel a reporter to testify and identify a source, though they ultimately backed down from that effort.

Sessions said the Justice Department must “balance the press’s role with protecting our national security and the lives of those who serve in the intelligence community, the armed services and all law abiding Americans.”

So far, the Justice Department under Sessions has publicly announced charges in just one leak case. Reality Leigh Winner, a 25-year-old government contractor, was charged in June with mishandling classified information after authorities said she gave a top-secret National Security Agency document to a news organization.

Trump’s presidency has been dogged by a steady stream of information provided to reporters by anonymous sources, though not all of those have involved classified information and many of the disclosures were likely not illegal.

Trump, for example, has complained that former FBI Director James Comey’s decision to engineer a leak of information about a conversation he had with the president was “illegal,” when legal analysts say that is not likely the case.

Comey has conceded publicly that he told a friend to give a reporter information about his recollection of the president’s request that he shut down the bureau’s probe into former national security adviser Michael Flynn. But he said he did not share classified material.

Prosecutors who bring charges against people for sharing information with the public can do so only when classified or other national security material is at issue. Material cannot be classified to conceal legal violations or prevent embarrassment, according to an executive order from President Barack Obama. Telling a reporter nonclassified information of public interest is not only legal, but it’s often the right thing to do.