The FBI told a judge they expected to find "evidence of obstruction" of justice in a search of former President Donald Trump's Florida home, according to newly released court papers.
Investigators said top secret files had been stored at Mar-a-Lago along with assorted newspapers and magazines.
The justice department said it censored the affidavit to protect "a significant number of civilian witnesses".
Mr Trump said the inquiry was being led by "political Hacks and Thugs".
On Friday, the US Department of Justice released a version of the affidavit used to justify the FBI's incursion into Mr Trump's Mar-a-Lago estate on 8 August, but it has been heavily redacted.
Given that the court papers were filed before the search of the Palm Beach estate, they contain no new details on the 11 sets of classified documents that the justice department has said were recovered from the exclusive golf club.
The FBI agent who drafted the affidavit wrote they had "probable cause" to believe that "evidence, contraband, fruits of crimes or other items illegally possessed" would be discovered.
"There is also probable cause to believe that evidence of obstruction will be found at the premises," the agent added.
The document shows that the unprecedented criminal investigation into a former president began after the National Archives, which maintains historic White House records, identified "a lot" of classified files among 15 boxes that it had recovered from Mar-a-Lago in January this year.
An FBI review of those materials uncovered 184 classified documents, including 25 marked as "Top Secret".
