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Officials at the Office of Emergency Management and Communications could not immediately say whether emergency workers at the center aided in that arrest but said they provided information that helped officers identify individuals who broke the law during Sunday’s protest.

If that kind of communication helped police, there was one incident in which a lack of communication led to what was perhaps one of the most ominous scenes of the weekend: those minutes when officer after officer could be seen putting on gas masks.

Superintendent McCarthy said what started as radio traffic of a single protester wearing a gas mask turned into talk among the officers that turned into what many thought was an order to prepare.

“I said, wait a minute — I’m the one who is supposed to give the order for gas,” Superintendent McCarthy said. And, he said, he hadn’t done that. “That was just a miscommunication.”

Not everybody agreed the police acted properly. Whatever violence there was, said Joe Iosbaker, a protest organizer, was the fault of police, not the protesters. And he tried to compare the events with what happened on the city’s streets in 1968. Dozens were arrested, and protest leaders reported a number of people injured.

“It’s not the same proportion, but the images have to be discussed in that context,” he said.

But other protesters didn’t see it that way. Liz Floyd, 26, of Ohio wondered why there were so many police on the street but did not think that those she saw used excessive force.

“I think they behaved overall,” she said.

The police were roundly praised by Mayor Rahm Emanuel, who had worked hard to lure the summit and likely had the most to lose if it did not go well. He said he personally thanked more than 400 officers.

“They did a tremendous job under very stressful circumstances over the last four days, and they make every one of us proud,” he said. “I wasn’t there, but I do know this: They showed the city and the country why they are the finest police department in the country.”

Mr. Futterman said that video shot by protesters, observers and the police department — and the many security cameras around the city — ultimately might show what mistakes police made.

But he said there is no disputing that police could have turned a tense situation into something much worse had they used their billy clubs more aggressively, used so-called sound cannons to create ear-splitting noise or doused protesters with pepper spray.

“They had all that (at their disposal), and they didn’t go there,” he said.

Associated Press writers Jim Suhr, Sophia Tareen and Ryan J. Foley contributed to this report.