BLOOMINGTON, Minn. - A janitor spots an abandoned diaper bag lying on a table in the sprawling food court at the Mall of America. A bomb-sniffing dog and a security officer are there within minutes, examining the package while nearby shoppers are held a safe distance away.

No bomb. Case closed. But that scene is repeated at the nation's largest shopping center 150 times a month.

Years ago, lost purses or shopping bags would just go to the lost and found. But after the Sept. 11 attacks and a series of terror threats against malls, "we realized that bad guys don't write 'explosives' on the side of packages," said Maj. Douglas Reynolds.

He heads a 150-officer security force trained in Krav Maga, the official self-defense system of Israeli security forces. A plainclothes unit is solely devoted to behavioral profiling.

Terror threats against U.S. malls - federal authorities have charged suspects in at least three terror plots since the Sept. 11 attacks - have made huge behind-the-scenes changes to one of the most treasured American experiences - going to the mall.

Shoppers say they hardly notice the closed-circuit cameras, plainclothes officers and trained dogs, and believe the risk of getting attacked at a shopping center.

"The average shopper, they don't walk in and think 'this could be the end,'" said Don Heinzman, 77, of Elk River, Minn., having coffee with two friends at the Minnesota mall.

But overseas, especially in places like Israel and Turkey, terror attacks in malls occur with frightening regularity. Experts are worried that similar acts will eventually become commonplace in the U.S. In 2004, an anonymous call threatening a Los Angeles plot sent more than 100 officers to protect various shopping centers.

Two Ohio men - originally from Somalia and Pakistan - are serving prison terms in a 2003 threat to bomb Columbus-area malls. Another suspect is serving a prison term for a similar plot against a mall 90 miles north of Chicago. A Massachusetts pharmacist is awaiting trial on terror charges; prosecutors said he conspired with others to shoot down shoppers in U.S. malls and kill U.S. troops in Iraq.

In a 2006 report, the nonprofit RAND Corporation think tank found that there were 60 shopping mall attacks in 21 countries between 1998 and 2005 and that U.S. malls may not be well-prepared for them.

The International Council of Shopping Centers trained some 10,000 mall officers between 2003 and 2009 to better recognize terrorists and other threats. Experts at George Washington University designed the $3 million program, which was discontinued because of a lack of funding.

Paul Maniscalco, a senior research scientist at the university who was involved in developing the program, called shopping malls "soft targets."

"I think they're as safe as any place else in the U.S.," he said. "Unfortunately in an open and free democratic society there's certain trade-offs. The concept of a shopping center is a pretty complex social icon within our society. You can't turn them into armed camps."

Malachy Kavanagh, the spokesman for the International Council of Shopping Centers, said the latest threat to public places in the U.S. is not from organized terrorist groups, but "lone wolf" individuals.

"A big part is to be aware of who may be watching your center," he said. Officers have to watch for people trying to engage guards in conversation, checking for security cameras, he said.

Reynolds said his officers need to cultivate a balance between securing a center and cultivating an open, family-friendly atmosphere (shoppers at the Mall of America can stop to ride rollercoasters at the mall's in-house amusement park).

"We're not designed to be Fort Knox," said Reynolds. "We need to be accessible and make people feel welcome - but still protected."

Christine Kimbrough 66, of Upper Marlboro, Md., stopped to look at a tall metal monument that resembles two doorways and a door.

The somber monument is a tribute to Bloomington resident Tom Burnett Jr., a passenger on United Airlines Flight 93, which crashed into a field in Shanksville, Pa., on Sept. 11, 2001.

Kimbrough read the monument's plaque and shook her head.

"You're always on alert," she said. "You have to be now."

Reynolds hopes shoppers think like Kimbrough. With 4.2 million square feet of space and more than 20,000 parking spots, it's difficult for his officers to see everything. He's instituted the so-called "RAM Unit" - short for Risk Assessment and Mitigation - which is a team of plainclothes officers who perform behavior profiling and who look for suspicious objects.

His officers don't carry guns but can make citizens' arrests under Minnesota law. There haven't been any terror arrests; most calls are about shoplifters, missing children and abandoned packages. Occasionally, the officers will confront a drunk and rowdy customer.

Reynolds said his officers must also be on the alert not just for terrorists, but for volatile workplace or domestic arguments that could result in a mass shooting.

The mall has a control center where dispatchers monitor 12 closed-circuit televisions and field the 120,000 calls for service each year.

Nearby, the bomb-sniffing dogs are in a separate office. Reynolds explained that he's transitioning from tough-looking Belgian Malinois dogs (similar to German Shepherds) to English Springer Spaniels and flat-coated retrievers, so that the dogs are perceived by shoppers as less aggressive and police-like.

"These dogs break hearts all day," he said, while patting Chuck, a four-year-old black and white spaniel.

If the client-friendly tactics sound like something out of Disney, that's because they are.

Reynolds has visited Orlando to learn from security experts there, and even uses some phrases similar to Disney's security force.

Officers who are patrolling the mall are "on stage," and inside the training room, there's a large word above the door that leads to the mall: "SHOWTIME."