Why was the MFA evacuated?

Larry Davidson was enjoying the Americans in Paris exhibit when the PA started blaring an emergency was underway and everybody had to leave the building:

... There were a lot of foreign visitors, many of whom were understandably confused. We got to the escalators, with a couple of hundred people ahead of us, and noticed that everyone was slowly lined up waiting patiently for the down escalator, while the up escalator was dutifully carrying nobody on its way upstairs. So we promptly pushed the red stop button and walked down, followed by half of other guests. Why didn't anyone else think of that? ...

Outside, he saw six fire trucks arrive.