Police Chief Calvin Williams issued a warning Tuesday to an undisclosed number of loosely-affiliated masked protesters who have triggered unusual anxiety among visitors and local residents with their unplanned appearances across the city: "If you are a member of a group that causes you to have to hide your face, then you probably need a different cause."

Protests outside the Republican National Convention have been peaceful, but police have received at least a dozen calls so far related to concerns about the small groups of black-clad anarchists who often don masks during demonstrations and other protests.

Following some of the calls, Williams said, officers have recovered slingshots and gas masks, both banned from the downtown event zone for the duration of the convention. That has prompted concerns about the group's intentions, Williams said.

"If you have a different issue (other than participating in peaceful protests), then we've got a problem," the chief said during a Tuesday morning briefing.

The pop-up groups, though small in number, appear to be the only disrupting force in a so-far uneventful series of protests that marked the convention's opening. http://www.usatoday.com/story/news/politics/elections/2016/07/19/cl...