US President Joe Biden said on Tuesday that he does not know what is in classified documents found late last year in an office he used at a Washington think tank. Biden also expressed his surprise at the discovery.

“People know I take classified documents, classified information seriously,” he told reporters in Mexico City, where he is on hand for talks with Mexican President Andrés Manuel López Obrador and Canadian President Justin Trudeau. “I was made aware of this discovery and was surprised to learn that there were government documents that were brought to that office. But I don’t know what’s in the documents,” said Biden, who pledged his full cooperation with the Justice Department’s investigation into the matter.

Biden occasionally used the think tank’s office between 2017 and the start of his 2020 presidential campaign. The contents of the ten documents, which were found in a closed cupboard by Biden’s lawyers who cleaned up his office, are as yet unclear. CNN reported that some of the documents are classified as “top secret,” meaning they contain sensitive intelligence.

Biden’s predecessor Donald Trump is facing legal problems because he took home more than 300 secret documents after his resignation. “When is the FBI going to raid Joe Biden’s many homes, maybe even the White House? Those documents have certainly not been released,” Trump said on his Truth Social platform on Monday, referring to the federal police search of his Mar-a-Lago estate in Florida.