CHICAGO (WBBM/CBS) — Mayor Rahm Emanuel made an unscheduled appearance at police headquarters Wednesday, and reporters wanted to know whether it was really flash mob gang activity – not cases of heat exhaustion – that closed North Avenue Beach earlier this week.

The purpose of the news conference was for Acting Police Supt. Garry McCarthy to introduce his No. 2 man, First Deputy Supt. Al Wysinger, who had been deputy chief of detectives.

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McCarthy said gang activity had nothing to do with the decision to close the beach on Memorial Day.

“No, that’s not what happened,” he said. “The folks who showed up at North Avenue, to what I was informed of, were suburbanites who were coming to the beach to enjoy a nice day. It was simply an overcrowding situation.”

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The announcement about a safety-related closure came over the public-address system shortly after 6 p.m. Monday. A firefighting crew sprayed crowds with a mist of water from a 2 1/2-inch hose as beach-goers made their way from the lakefront.

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Mayor Emanuel said Tuesday that the closure was about public health and public safety. He said police had to shut down the beach “so the paramedics could do what they needed to do,” given the number of calls for heat exhaustion.

In all, four people were rushed to area hospitals. One was even listed in critical condition.

But people who were at the beach said there were roving groups of menacing young people dressed in gang members’ choice of clothing – oversize white T-shirts and pants hanging down.

But McCarthy insisted there was no gang activity.

Violent flash mobs have been a concern at North Avenue Beach previously.

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In April at the beach, sources say a group of about 100 teens surrounded two people on bikes. Both were knocked off their bikes and thrown into Lake Michigan.