The question hanging over the mountain of paperwork after FBI agents carted away about a dozen boxes of presidential records from Donald Trump’s opulent Mar-a-Lago residence is: What kind of criminal case is the Justice Department building against him?

Federal investigators are looking into whether Trump violated a law requiring him to turn over almost all of his White House documents to the National Archives, as well as whether he mishandled classified documents discovered at his Palm Beach resort.

Legal experts, including former federal prosecutors, say the Justice Department and FBI would never have obtained a search warrant and carried out such an unprecedented raid on a former president’s home on Monday unless Trump was suspected of committing a crime or allowing classified national security documents to circulate at Mar-a-Lago.

“There is no doubt that they used a search warrant because the government was not getting the information they were requesting from Donald Trump,” said Mark Schnapp, a longtime Miami attorney who previously worked as a federal prosecutor on public corruption and financial fraud cases.

Christina Bobb, one of Trump’s lawyers, told Right Side Broadcasting Network that the raid on Mar-a-Lago “came as a surprise.” She stated that the former president “has been very cooperative” with the FBI and Justice Department, even allowing agents to visit the premises in June to discuss classified materials that needed to be stored in a secure location.

According to law enforcement sources familiar with the raid, the FBI’s search warrant, signed by federal Magistrate Judge Bruce Reinhart in West Palm Beach, stated that Trump had failed to turn over his presidential documents. The documents included classified materials that the National Archives and Records Administration has attempted to obtain from Trump since he and his lawyers initially turned over 15 boxes of sensitive documents to Mar-a-Lago in January.

The Presidential Records Act applies to Trump and his predecessors in the White House. The law was enacted in 1978, after former President Richard Nixon attempted to destroy White House recordings that documented activities related to the Watergate scandal.

According to a Congressional Research Service report, when a president leaves office, the archivist takes custody of the records from that administration and is responsible for their preservation and public access.

The law, according to Jason R. Baron, a professor at the University of Maryland’s College of Information Studies and former director of litigation at the National Archives, is clear.

Anyone bringing charges will “have a difficult time proving that he actually knew anything was in the boxes or anywhere else,” according to Bobb.

It is not the first high-profile FBI investigation into a potential record-keeping violation. Former Secretary of State Hillary Clinton’s use of a private server for government work was the subject of an extensive federal investigation in 2016, which consumed her presidential campaign and, according to her, may have cost her the election to Trump.

Clinton was eventually accused of being “extremely careless” in her use of private email by the FBI director, then James Comey. However, the agency found no criminality and did not recommend charges against her.

In response to Clinton’s email practices, Republican lawmakers increased the penalty for unauthorized removal of classified material from a misdemeanor to a felony — a punishment that now awaits the former president.

During the House Oversight and Reform Committee’s investigation of the FBI’s probe into Clinton’s use of email, Valerie Shen, the Democrats’ chief national security counsel, said the FBI probe likely arose from another investigative body realizing that important documents were still missing, even after the National Archives recovered 15 boxes of material from Trump earlier this year.

Bobb, who was at Mar-a-Lago during the FBI search, told far-right media on Tuesday that she and the former president’s other attorneys had already reviewed all of the classified documents stored in Trump’s personal home before they were repossessed by law enforcement Monday night.