AURORA — A $70 alarm on the exit door in theater 9 might have prevented a mass shooting at the Century Aurora 16 multiplex.

But in today’s world of the multiplex, moving large crowds in and out of the building efficiently is paramount, and those exit doors play a key role in clearing a theater quickly.

Doors leading from a theater auditorium to the outside, such as the one police say was utilized by James Eagan Holmes to slip in and kill 12 and injure 58, are not only common at most multiplex theaters worldwide but are also required by most fire codes. And while there is neither prohibition against putting alarms on them nor requirement to do so, many theater chains eschew them and instead hope some patrons exit that way to clear the room quickly and prepare for the next showing.

“It depends on whether they really want to force their audience to come back through the lobby to exit,” said Erich Friend, a consultant on theater and performing-arts safety. “That can create kind of a traffic jam.”

Through a spokesperson for its attorney, Cinemark, operator of the Century Aurora 16, declined to comment on whether the rear door that police say Holmes used was alarmed but disabled, but there is no indication it was. At another Cinemark theater, the Century 16 Bel Mar in Lakewood, the doors leading to parking lots are not alarmed and remained that way last week.

Bel Mar’s theater 9 has a door to the side of the screen leading to the parking lot, similar to what appears to be the arrangement at the Aurora multiplex. That would be in keeping with the international fire code used by Aurora and many other cities, which requires two exits from a room holding up to 500 patrons.

The capacity of Aurora’s theater 9 has not yet been released. Attorneys hired by Aurora have cited a broad interpretation of a judge’s gag order as reason to refuse requests from The Denver Post for copies of the building plans for the Century 16 there.

Theater 9 at the Lakewood multiplex seats a maximum of 432. Under the formula required by the international fire code, which calls for one-fifth of an inch of exit width for every occupant, that capacity requires a little more than 7 feet of total exit openings to get them out. Some head toward the lobby, some directly outside.

Those exterior doors must remain unlocked from the inside, though they can be locked to the outside. Police believe Holmes went undetected by theater personnel as he propped open the door while he went to his car, got his guns and other equipment and returned.

“The exit doors should not be locked,” said Capt. Keith W. Dix, deputy fire marshal for West Metro Fire Rescue, which covers the multiplex at Bel Mar. “The concern is not about people getting in. Our No. 1 concern is making sure people can get out.”

Theater companies have known for decades that exterior doors are convenient entry points for thieves. One friend buys a ticket, then props or opens the door so his or her buddy can get in for free.

It’s as old as the multiplex itself and in such common usage that options for accomplishing an unlawful sneak-in are outlined online on blogs such as moviecultists.com, which covers the topic in a post called: “How to sneak into a movie: 3 easy methods.”

“While the front entrance is usually crawling with theater employees, these exit-only doorways are, more often than not, completely void of anyone nearby,” the post notes. “So go in, buy a ticket, walk to the exit-only doorway, open it, and let all your friends in.”

In 1997, the industry learned that a propped-open exit door can pose a much greater threat than just theft.

In a North Vancouver, British Columbia, incident that bore disturbing similarities to the Aurora theater shooting, a man dressed in a disguise burst into a theater from a propped-open exterior exit door, then opened fire. But he was interested only in killing a rival gang member and did not spray the theater with bullets.

Just as was initially true for some in the Aurora case, patrons believed the gunfire was part of the film.

The shooter was later caught and sentenced to a minimum of 25 years, but his actions did not lead to changes in the theater industry, such as door alarms.

Those who use them today do so not to prevent the unthinkable like the Aurora massacre but just to keep the place secure.

Jeff Logan owns Logan Luxury Theaters Corp. — three movie theaters and a drive-in in three South Dakota towns. He has always had alarms.

“In the early days,” he said, “it prevented a kid from paying for a ticket and coming in and then letting 10 of his buddies in who were waiting in the alley.”

It also prevents teenagers from opening the door to sneak in beer and other alcohol.

“The alarms are effective at preventing all sorts of mischief,” Logan said. “We know when it goes off, somebody is up to something.”

Chuck Murphy: 303-954-1829, cmurphy@denverpost.com or twitter.com/cmurphydenpost