See how Pokémon Go works, and why everyone's so crazy about it. (Video: Jhaan Elker/The Washington Post)

Three people who had been playing the wildly popular cellphone game Pokémon Go were robbed of their phones Tuesday night at the University of Maryland campus in College Park.

Outside St. Louis, authorities in O’Fallon, Mo., are investigating how a band of four robbers, armed with at least one handgun, targeted Pokémon Go players.

In Austin, a man reportedly was robbed of his wallet at gunpoint at 1:23 a.m. Tuesday while playing Pokémon.

The incidents have police departments across the country warning gamers not to bury their heads in their phones. “You need to be aware of your surroundings and be careful,” said Marc Lovicott, communications director for campus police at the University of Wisconsin at Madison.

Lovicott and other officials also are worried about distracted gamers walking into traffic, while safe-driving advocates are warning people not to play the game while behind the wheel.

New mobile game ‘Pokémon Go’ takes the world by storm

epa05421306 Two high school students play with the new game 'Pokemon Go' on their smartphones on Flinders Street in Melbourne, Victoria, Australia, 12 July 2016. Pokemon Go, a Global Positioning System (GPS) based augmented reality mobile game, is proving to be 'enormously' popular since software development company Niantic opened access to it on 06 July in the US. EPA/JULIAN SMITH AUSTRALIA AND NEW ZEALAND OUT (Julian Smith)

An "augmented-reality" game, Pokémon Go prompts players to walk around and try to find cartoon creatures seen on their phone screens. The virtual images are overlaid on real surroundings.

In O’Fallon, authorities suspect that the thieves could have used the Pokémon Go app to find the players. “If you use this app — or other similar type apps, or have children that do — we ask you to please use caution when alerting strangers of your future location,” O’Fallon police said in a news release Sunday.

“It is unfortunate that people trying to play a game have to worry about being targeted by assailants,” Randy Burba, the chief of public safety at Chapman University in Orange, Calif., said in an email Wednesday.

Burba pointed to a specific feature of the game: PokéStops, which attract various users to the same place. “These are typically located at churches, retailers, public locations, monuments and other attractions,” Burba wrote. “Since they are more attractive, predators can stake out at or near a PokéStop and commit crimes against other users. This goes back to being aware of your surroundings. Best to visit PokéStops at day time, with a friend or in a group.”

The same caution extends to PokeGyms, he said, which by design attract players who meet to play against others.

At the U-Md. campus in College Park, police said that there were three robberies over about an hour Tuesday involving a total of four victims.

Sgt. Rosanne Hoaas, a spokeswoman with U-Md. police, said one student was playing Pokémon and two others had just finished playing when they were robbed. A fourth person — who wasn't playing Pokémon but was using a GPS app on his phone to walk through the campus — also was robbed of his cellphone during that time. The victim of the fourth robbery was not a student.

The robberies have prompted police to warn people playing the game to do so with a friend and stay in well-lit areas.

The thrill of the hunt near Texas A&M University led a driver to illegally park a car — to “fetch” a Pokémon — whereupon the car was struck from behind by another vehicle.

The game also could be a danger to distracted pedestrians who wander into a busy street.

Pokémon Go’s unexpected side effect: Injuries

In Madison, the official Twitter account of the campus police warned players to watch where they walk. “Pokemon aren’t worth it,” the tweet read.

Lovicott, who sent the tweet, said the college town had experienced a few “close calls” with pedestrians and bicyclists veering into traffic. “We don’t want people walking into roadways playing this game,” he said.

A statement from AAA warned drivers to not catch Pokémon and drive at the same time. “Trying to catch a Pokémon while behind the wheel is a major distraction and increases your risk of causing a crash,” AAA Georgia spokesman Garrett Townsend said. “Put your phone down while driving. Just one quick text or glance to see the next Pokéstop could end up costing you or someone else their life.”

At U-Md., the first incident occurred at 9:08 p.m. outside Tydings Hall, where the suspect implied that he had a weapon. As police were searching the area for a suspect, they said they received a call with more reports of robberies.

The second incident occurred about 10:12 p.m. near Queen Anne’s Hall, and the third three minutes later, also near that hall. In both incidents, police said, a black handgun was seen.

Three of the four victims are students at the university, police said. No one was hurt. Officers searched the area and could not find a suspect described as a man wearing a black mask and dark clothing. Anyone with information is asked to call 301-405-3555.

Dana Hedgpeth and Victoria St. Martin contributed to this report.