International Monetary Fund chief Christine Lagarde has said the decision to formally investigate her over negligence claims was "without basis".
In a statement after a fourth round of questioning before magistrates, Lagarde said she was returning to work in Washington later in the day.
"After three years of proceedings, dozens of hours of questioning, the court found from the evidence that I committed no offence, and the only allegation is that I was not sufficiently vigilant," Lagarde said.
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Williams is charged with first-degree murder without premeditation, a probation violation and for not having a valid driver’s licence.
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The Foreign Secretary is a long-term opponent of a third runway being built at the west London airport, which the government supports.