(CNN)A ninja museum in central Japan had some stealthy visitors this week -- thieves who broke in and stole more than a million yen ($9,470) in the middle of the night.
The Iga-ryu Ninja Museum, located in Iga city in Mie Prefecture, is dedicated to the history and practices of ninja. It notified police after an alarm went off in the early hours of Monday morning.
At the time, there were no staff at the museum, which is a popular tourist site. When the police arrived, they found the museum entrance had been forced open, and the safe containing the money was missing.
The safe, which weighed about 150 kilograms (330 pounds), held admission fees from more than 1,000 visitors, according to the museum.
"It was a three-minute job," said an official at the museum, who requested anonymity for privacy reasons. "It was planned, they must have scoped us out and singled us out."
Part of the museum's appeal is that it's tucked inside a forest -- but this also makes it a better target for thieves, as it's largely hidden from view once night falls.
The museum's security cameras showed a car pulling up to the building on the night of the robbery, and a man climbing out of the passenger seat. He walked toward the camera, and tilted it down so it only filmed the ground for the rest of the night.
The official added that the heist had occurred just as visitors were beginning to return to the museum over the summer holidays.
"There's a second wave of the (coronavirus) now, but people were just getting more comfortable with all the corona precautions we were taking. This is really terrible," he said.
Known for their secrecy and high levels of skill, ninjas were masters of espionage, sabotage, assassination and guerrilla warfare dating back to at least the 14th century. They were fabled as hired assassins, and have been steeped in mystery and lore for centuries.
Now, they're an established part of Japan's tourism and entertainment industries; theater and pantomime groups often perform ninja acts and scenes in full costume.
Iga, the city where the museum is located, is known as a center for ninja enthusiasts; according to local legend, it was once home to the famous Iga clan of ninjas.
Every year, Iga holds a massive ninja festival that attracts thousands of attendees with traditional performances, ninja costume stations, ninja training sessions and more.
In 2017, Mie University established the International Ninja Research Center in Iga -- the first in the world dedicated to ninja studies.
Genichi Mitsuhashi, 45, became the first person ever to hold a master's degree in ninja studies after completing Mie University's graduate course last year. He now teaches the art of ninja in his own dojo in Iga.
"Iga is where ninja used to live," he told CNN in June. "The climate of this area created the very nature of ninja."