(CNN) — President Donald Trump’s national security adviser, Robert O’Brien, has tested positive for Covid-19, according to an official familiar with what happened. O’Brien’s diagnosis marks the highest-ranking Trump administration official known to have tested positive. It’s unclear when O’Brien last met with Trump. Their last public appearance together was over two weeks ago during a visit to US Southern Command in Miami on July 10. O’Brien, one of Trump’s top aides, recently returned from Europe, where he and his top deputy met with officials from the UK, France, Germany and Italy. A senior administration official told CNN that O’Brien has been working from home since last week.