The FBI search of former President Donald Trump's Mar-a-Lago residence was sparked by an "FBI confidential human source" in the Trump camp who knew what documents to look for and the location.
According to two separate reports in Newsweek and the Wall Street Journal, the raid of Trump's Florida residence was also "deliberately timed to occur when the former President was away".
Two dozen FBI agents and technicians arrived at the Florida home to execute a search warrant on Monday in an investigation believed to involve provisions of the 1978 Presidential Records Act.
If Trump is found to have violated federal law in removing and retaining classified documents he could be convicted of a felony punishable by five years in prison under laws he signed in the wake of a furore over Hillary Clinton's use of a private email account.
According to the Wall Street Journal, "someone familiar with the stored papers told investigators there may be still more classified documents at the private club" and the material was beyond that handed over to the National Archives earlier this year.
But amid a political furore over the raids, a furious FBI official, who spoke to Newsweek after requesting anonymity, said attempts to avoid a media circus over the raid had "backfired".
"I know that there is much speculation out there that this is political persecution, but it is really the best and the worst of the bureaucracy in action," the official told Newsweek.
"They wanted to punctuate the fact that this was a routine law enforcement action, stripped of any political overtones, and yet [they] got exactly the opposite."
The second source, a senior intelligence official who was briefed on the investigation and the operation, said: "They were seeking to avoid any media circus.
"So even though everything made sense bureaucratically and the FBI feared that the documents might be destroyed, they also created the very firestorm they sought to avoid, in ignoring the fallout."
In February, presidential archivist David Ferriero testified before Congress that the Trump camp had already returned 15 boxes of documents to the Archives.
He said the boxes included items "marked as classified national security information".
According to news reports, a further 10-15 boxes of documents were removed from the premises during this week's raid.
"This unannounced raid on my home was not necessary or appropriate," former President Trump said in a statement.
Presidential historian Robert David Johnson told the Washington Post that every President has violated the Presidential Records Act in some way, such as by using personal phones for texts or emails, for example.
"But Trump might be the most egregious violator of the law in its 44 years of existence, Johnson said.
"Since [Richard] Nixon, there is no example of a President just pretending the law doesn't exist."
However, supporters including former Trump administration official Kash Patel are now arguing Trump declassified the material he took to Mar-a-Lago.
"What I can tell you definitively is that President Trump was a transparency President," Patel said.
"And President Trump, on multiple occasions at the White House, declassified whole sets of documents. Including — I remind you and your audience that around October of 2020, he issued a statement from the White House declassifying every document related to not just the Russiagate scandal, but also the Hillary Clinton email scandal."
Previously, Patel argued that even if the White House failed to generate the paperwork it didn't mean the material was not declassified.
"Trump declassified whole sets of materials in anticipation of leaving government that he thought the American public should have the right to read themselves," he said. "The White House counsel failed to generate the paperwork to change the classification markings, but that doesn't mean the information wasn't declassified."
Meanwhile, a separate unrelated civil investigation is probing whether the Trump Organisation inflated real estate values to obtain favourable loans and understated the values to get tax breaks.
The former President has denied any wrongdoing and condemned the investigation as politically motivated.
"I declined to answer the questions under the rights and privileges afforded to every citizen under the United States Constitution," Trump said in a statement.
"I once asked, 'If you're innocent, why are you taking the Fifth Amendment?' Now I know the answer to that question.
"When your family, your company, and all the people in your orbit have become the targets of an unfounded politically motivated witch hunt supported by lawyers, prosecutors and the fake news media, you have no choice."
The Fifth Amendment establishes a number of rights related to legal proceedings, including that no one "shall be compelled in any criminal case to be a witness against himself".