The NY Times looks at the threat of terrorism in Rio:
http://www.nytimes.com/2016/08/02/world/americas/rio-de-janeiro-olympics-terrorism-brazil.html?hp&action=click&pgtype=Homepage&clickSource=story-heading&module=second-column-region®ion=top-news&WT.nav=top-news
Here are some American soldiers before training the Brazilians: