What investigators needed to get a search warrant for Trump's home, according to experts

Donald Trump's Mar-a-Lago estate was searched by federal agents on Monday morning in what an outside expert called a "major escalation" of one of the investigations he faces.

Sources told ABC News that the raid, carried out by the FBI, was related to the 15 boxes of records that the former president took to his Florida home when he left the White House.

Trump, who was not present for Monday's search, said in a statement that the agents obtained access to his safe as they executed the search warrant. Sources told ABC News that the safe referenced was in Trump's office on the compound; people close to Trump said that agents did not ask for the code and instead broke it open.

Law enforcement experts unconnected to the case called the raid a notable step forward in the federal probe -- which appears to trace back to a National Archives referral to the Department of Justice reported early this year -- and they explained the significance of federal search warrants and what they indicate about the general timeline of a case.

"[The search is] absolutely unprecedented and is a major escalation," Neama Rahmani, a former federal prosecutor and president of West Coast Trial Lawyers, told ABC News.

"We haven't heard much from [Attorney General] Merrick Garland in the past year and a half, but this is a clear indication that the Department of Justice is going to move forward," Rahmani added.

The DOJ opened the grand jury investigation after National Archives officials confirmed in a letter to the House Oversight Committee that some of the documents recovered from Mar-a-Lago were marked classified. The National Archives referred the matter to the attorney general.

While little has been reported publicly about the probe, in keeping with department policy, experts said that in order to obtain a search warrant, investigators needed to have probable cause of a violation of federal law.

That means authorities would need to prove that there is sufficient reason based on known facts to believe that a crime has been committed, or that a certain property is connected with a crime.

The raid, however, doesn't mean prosecutors have determined Trump committed a crime. In his statement, he labeled Monday's raid an act of political persecution.

While probable cause is a lower legal standard than beyond a reasonable doubt or preponderance of evidence, experts said it's likely the federal case is airtight given the gravity of raiding the residence of a former president.

"This thing would have to be bulletproof," Nick Akerman, a former Watergate special prosecutor, told ABC News.

"This would be a very detailed affidavit that would almost present a case beyond reasonable doubt, such that there's no way you could ever fault a judge or a prosecutor issuing this warrant," he said.

The issuance of a federal warrant also likely means investigators received new information justifying such a seizure.

"The probable cause cannot be stale," Akerman said. "This can't be based on something that happened 18 months ago when Trump left office, it has to be something that happened more recently."

In response to the warrant, many leading Republican have echoed Trump in accusing the DOJ of being politically motivated. Some invoked the looming November midterm elections.

"The Department of Justice has reached an intolerable state of weaponized politicization," House Minority Leader Kevin McCarthy, R-Calif., said in a statement. "When Republicans take back the House, we will conduct immediate oversight of this department, follow the facts, and leave no stone unturned."

Trump said the search was an "an attack by Radical Left Democrats who desperately don't want me to run for President in 2024."

Generally speaking, the DOJ avoids major actions involving political candidates in the months just before an election. But that rule is not written into law -- as was seen when the FBI publicly commented on its investigation into how Hillary Clinton handled classified material in the final days of the 2016 presidential race -- and Trump has not declared he is running again in 2024.

The indication that the FBI search was in relation to Trump's handling of government files has raised questions about one statute in particular: Section 2071 of Title 18 of the United States Code.

The law states that anyone who "willfully and unlawfully conceals, removes, mutilates, obliterates, falsifies or destroys" government records faces a fine or up to three years in prison and is "disqualified from holding any office under the United States."

But legal scholars have long been torn about whether the statute, which was also in the spotlight when Clinton used a private email server to conduct government business while secretary of state, could actually bar Trump from seeking another term given that the Constitution sets out the sole qualifications to be president.

ABC News' Katherine Faulders and John Santucci contributed to this report.