As the nation reels from the horror of the early morning shooting of 71 people in an Aurora, Colorado theater by what appears to be a lone gunman, law enforcement agencies across the country are responding with caution and asking the public to be more vigilant.

Suspect James Holmes, identified as the suspect in the Colorado shootings, in a photo from the University of Colorado

“While the tragedy in Aurora, Colorado appears to be an isolated incident, our vigilance has been raised,” said Captain Mike Parker, of the Los Angeles County Sheriff’s Headquarters Bureau. “The heightened alert of the Los Angeles County Sheriff’s Department includes increased patrols to create a more visible presence at movie theaters and other places where people congregate.”

“As always, we encourage the public: ‘If You See Something, Say Something.’ The greatest deterrant to crime is often a phone call to law enforcement by a person who sees something that just doesn’t look right.”

“Our thoughts and prayers go to the many victims, families, friends, and first responders affected by this senseless crime.”

Calls to the Regal/Edwards Theater administration regarding any local changes to scheduled showings were not returned. (This story will be updated as details become available).

Brave New World, a comic book store in Newhall, had scheduled a group showing tonight, but is offering refunds to any ticket holders who want to skip the festivities. They posted the following on their website:

“Brave New World Comics would like to offer our deepest condolences to the people and families of those in Aurora, Colorado for the horrific events that occurred this morning.  In light of these events, if anyone with tickets to our screening of The Dark Knight Rises feels that they would rather not see the movie tonight, please contact us, and we will arrange a refund.

NBC News offers a complete report on the incident:

By Pete Williams and NBC News Staff

A black-clad gunman wearing a gas mask opened fire on the audience in a scene of “mass chaos” at a midnight showing of “The Dark Knight Rises” near Denver on Friday, killing 12 people and wounding 59 others, many of them critically, authorities and witnesses said.

The apartment of the suspect, identified as James Eagan Holmes, 24, had been booby-trapped with what police described as sophisticated explosives or flammable materials, and officers were trying to determine how to defuse them, Aurora Police Chief Dan Oates said. The area was evacuated, and police were expected to remain on the scene “for hours or days,” he said.

Oates initially said 14 people had been killed, but the figure was later revised to 12.

Holmes, a graduate student from San Diego who was in the process of withdrawing from the neuroscience program at University of Colorado-Denver medical school, put up no resistance when he was arrested in a parking lot at the theater, police said.

“We are confident he acted alone,” Oates said of Holmes, who was scheduled to appear in court Monday to face unspecified charges.

The victims, who were being treated in six hospitals, included a 6-year-old. A 4-month-old baby also was treated and released. The oldest reported patient is 45.

One of those killed was Jessica Ghawi, a sportswriter who survived a June 2 mass shooting at a mall in Toronto in which two people were killed and seven others were injured. Ghawi blogged under the name Jessica Redfield.

Buckley Air Force is in Aurora, and defense officials told NBC News that two Air Force reservists and two Navy service members were among those shot. Their identities and conditions weren’t available, but defense officials said none was killed.

Authorities said the gunman appeared at the front of the theater at 12:39 a.m., about 20 minutes into the film, and released a canister of tear gas. Witnesses told reporters that the gunfire erupted during a shootout scene.

“It was mass chaos,” witness Jennifer Seeger told TODAY. The gunman shot the ceiling and then “he threw in the gas can, and then I knew it was real.”

“I told my friend, ‘We’ve got to get out of here,’ but then he shot people trying to go out the exits,” Seeger said. She said the shooter made his way up the aisle, shooting as he went, saying nothing.

Witnesses describe what they saw at the theater in Aurora, Colo.

The shooting occurred in the Century 16 Movie Theaters at Aurora Town Center. Aurora is a suburb less than 10 miles east of downtown Denver.

NBC station KUSA of Denver cited a witness as seeing a black-clad, 6-foot-tall man wearing a riot helmet, goggles and a bulletproof vest. Witnesses said the gunman entered the theater through an emergency exit door.

The suspect was found in possession of a gas mask, Oates said. Ammunition was found in the suspect’s car, police said. Initial reports the car had Tennessee plates turned out to be wrong.

Former FBI profiler Clint Van Zandt tells Daily Rundown’s Chuck Todd “this was a pre-planned event.”

Oates said the gunman had four weapons: two 40-caliber Glock handguns, a Remington 870 single-barrel shotgun and a Smith and Wesson AR-15 assault-style rifle. Three of the weapons were found in the suspect’s white Hyundai; one of the handguns was found in the theater.

Oates didn’t say what kind of magazines were used, but “many shots were fired.” Some rounds penetrated an adjoining theater and injured at least one person, he said.

James Yacone, the FBI’s agent in charge in Denver, said there was no early indication of a link to terrorism. Holmes wasn’t on any federal law enforcement watch lists, authorities told NBC News, and Oates said he had no police record beyond a speeding ticket last year.

In his apartment rental application early last year, Holmes described himself as a “quiet and easy-going” student.

A pharmacy student who lives in the building, which is reserved for students, faculty and staff of the medical campus, described him as a recluse.

“No one knew him. No one,” the man, who asked to be identified only as “Ben,” told The Denver Post.

The University of California-Riverside confirmed that a man named James Holmes graduated from the university with a degree in neuroscience in 2010. His last known address was in San Diego, it said.

Gov. John Hickenlooper said at a news conference that “our hearts are broken as we think of the friends and family of the victims of this senseless tragedy.” He called the shootings “the act of an apparently very deranged mind.”

“The pain and grief (are) too intense for words, but we can’t let it keep us from our lives,” Hickenlooper said. “We are going to come back stronger from this but it is obviously going to be a very long process.”

President Barack Obama cut short a campaign visit to Florida to return to Washington ahead of schedule.

He called for reflection after the attack. “There are going to be other days for politics,” Obama said during an abbreviated appearance in Fort Myers, where he led a moment of silence on behalf of the victims and their families.

At a campaign appearance in Bow, N.H., Republican presidential challenger Mitt Romney said: “Today is to remember and reach out and remember our blessings in life. Each of us will hold family close and spend a little less time thinking about worries of day and helping those in need of compassion.”

Police raiding the theater in the hunt for the suspect had to ask for gas masks.

“Get us some damn gas masks for theater 9. We can’t get in it,” one officer radioed back to emergency dispatch during the operation, according to an excerpt aired on KUSA.

Moviegoers described scenes of chaos and terror inside the movie theater.

Tanner Coon, 17, describes seeing flashes of gunfire, which he thought were fireworks, amid the chaos of trying to escape the shooting as he was “trying to calm” his friend’s 12-year-old brother.

Tanner Coon, who was in the theater with a friend and the friend’s 12-year-old brother when the shooter came in, said he told them to “get down” when he heard the gunshots.

The shooter fired off about 20 rounds and there was then a pause and a “period of quietness when everybody started running out,” Coon said.

“I slipped on some blood and landed on a lady. I shook her and said, ‘We need to go.’ There was no response, so I presume she was dead,” Coon said.

At least three people had been treated for chemical exposure, KUSA reported.

Friday’s incident was the worst mass shooting in the U.S. since the 2007 shooting on the campus of Virginia Tech University in Blacksburg, Va., in which 33 people died, including the gunman.

It was the deadliest mass shooting in Colorado since the Columbine High School massacre on April 20, 1999 in the Denver suburb of Littleton, just 15 miles west of Aurora. Twelve pupils and a teacher were killed and 26 other people were wounded before the two gunmen killed themselves in the school’s library.

Reviews of “The Dark Knight Rises,” the third installment of the Batman franchise, have noted its dark, anxiety-fueled themes, which reminded some of the atmosphere in the days after the terrorist attacks of Sept. 11, 2001. It had been one of the most eagerly awaited films of the year, and theaters around the world began showing it at 12:01 a.m. Friday.

Warner Brothers has cancelled the Paris premiere of the film, which was scheduled for this evening.

“Warner Bros. and the filmmakers are deeply saddened to learn about this shocking incident. We extend our sincere sympathies to the families and loved ones of the victims at this tragic time,” the studio said in a statement.