Mueller thought that a 'thorough' FBI investigation into Trump would reveal other crimes

According to special counsel Robert Mueller's report, President Donald Trump would be in big trouble if the FBI ever did further investigations into him or the campaign.

"The president wanted to protect himself from the investigation into his campaign," the Mueller report revealed on page 76. "The president had a motive to put the FBI's investigation behind him. The evidence does not establish a determination if Comey was designed to cover-up a conspiracy between the Trump campaign and Russia."

However, further investigation might have uncovered crimes.

"The evidence uncovered did not establish that the president was involved or those close to him were involved in the computer hacking or active measure conspiracies or that the president are an unlawful relationship with any Russian official," the report continued. "But the evidence does indicate that a thorough FBI investigation would uncover facts about the campaign and the president personally that the president could have understood to be crimes or that would give rise to personal and political concerns."

It seems Trump was concerned that if the FBI did any further investigation into the Trump campaign or the president, they would have uncovered other crimes.