Several hundred people gathered at a vigil Friday night near the movie theater where 12 people were killed and 58 injured in a shooting rampage. Police were planning to try to enter suspect James Holmes's apartment, which he told authorities he had booby-trapped with explosives.
New York Police Commissioner Ray Kelly said Friday extra officers were on duty in and around the city's movie theaters in an effort to prevent copycat shootings, CNN reported.
"We're just concerned that someone perhaps seeking notoriety will attempt to do something similar," he said. "We always hear that when a high-profile event happens, so we're doing this to sort of raise the comfort level of people who are going to the movies."
Similar precautionary police efforts were taking place in Los Angeles, Dallas, New Orleans, Tampa, Fla., Washington, D.C., and Chicago.
"It's a little bit of a loss of innocence," Chris Johnson, vice president and co-owner of Classic Cinemas, in Chicago, told the Chicago Tribune.
"It's sad," he said. "We're such a great industry where people go out just to have fun."
Meanwhile, theater chains and owners are considering implementing permanent security measures, said Gary Kline, spokesman for The National Association of Theatre Owners.
"At this moment we're just responding to what happened and we'll be talking to our members and it's up to individual members," Kline told NBC News.
AMC Theatres told Deadline Hollywood it plans to reinforce its rules preventing guests "in costumes that make other guests feel uncomfortable and we will not permit face-covering masks or fake weapons inside our buildings."
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