For President Donald Trump and his supporters, the news that Special Counsel Robert Mueller had come to a close with no new indictments has been reason to celebrate the end of a trying 22 months. Mueller’s report, as summarized by Attorney General William Barr, did not say Trump had not acted to obstruct justice during the course of the probe, but the president and his Administration have nevertheless said that it amounts to a “total exoneration.”

“We’re glad it’s over,” Trump said on Monday.

But experts on the history of Mueller’s task tell TIME that, while his job was to focus solely on proving whether a law was broken, it can be risky for Congress and the American people not to hold politicians to a higher standard.

In scandals going all the way back to the era of Ulysses S. Grant and the very first special prosecutor — an office that has gone by a number of names over the years — they see moments when the end of an investigation has been used to excuse other problems in government. And as the nation faces the question of what comes after the Mueller report, they see American history as a strong argument for continued vigilance.

“There are plenty of cautionary tales strewn throughout the history of American special prosecutors,” says Andrew Coan, author of Prosecuting the President: How Special Prosecutors Hold Presidents Accountable and Protect the Rule of Law.