Koi fish are seen in a Washington pond in this 2011 file photo. Police say about 400 similar fish were taken from a pond earlier this month in Fairfax County. (Bonnie Jo Mount/The Washington Post)

The great koi heist began with the thieves handing out a business card.

The two neatly dressed men in khakis and white shirts showed up at a business park in the Herndon area in early June, saying they were with an aquatic-care company and had come to remove sick fish from a pond, one person familiar with the incident said.

They spoke knowledgeably about koi and worked diligently, using large nets over four days to haul in the brightly colored fish that are popular with the Dell and Northrop Grumman workers there who eat lunch outside.

The criminals’ ruse was so well orchestrated that no one realized 400 koi had been carefully packed in large coolers and stolen until after the men were gone and security mentioned the crew to the property-management company. An even greater shock: The fish might be worth tens of thousands of dollars.

“We thought, ‘Wait a minute. No one was hired to look at the fish,’ ” said an employee at the office park who was not authorized to speak to the media. “We were stunned. We were shocked. Who steals these kind of fish?”

Fairfax County police are still trying to solve the mystery, but the strange case has opened a window on the little-known and arcane world of koi collectors, who will pay as much as $25,000 for a championship fish and passionately pit their prized specimens against each other at competitions.

There have been scattered reports of koi thefts across the country in recent years.

“There is a whole little cult around this fish,” said Steve Maletzky, the owner of Tropical Lagoon Aquarium in Silver Spring. “It’s almost like dog shows.”

Koi, or carp, vary widely in price and are beloved for their riot of oranges, yellows and blacks and their distinctive slash and spot markings. A conservative estimate of the value of the 400 stolen fish is near $20,000, but they could have been worth far more if many of them were large, said Philip Gray, president of the Mid-Atlantic Koi Club. A koi of 18 inches can fetch about $2,000, he said. Young fish retail for about $8.

Police said the men, in their 50s, first showed up at Dulles Corner Park on June 8. The men said they worked for DSC Aquatic Solutions and handed security officials a business card, the person who works at the business park said.

David Cutlip, the owner of DSC, said that his company manages the ponds at Dulles Corner Park but that the men did not work for him and that it was a mystery how they might have obtained a business card from his company.

“I was floored when I heard,” Cutlip said.

The suspects were described as white and were wearing sunglasses. One was overweight and about 5-feet-5 inches.

Nothing seemed amiss as the men brazenly began their work on the small pond, which is surrounded by trees and office high-rises, the person said.

The koi, which had been in the pond for about 25 years, were a treat for employees.

“We love to go there and relax and eat our lunch,” Amber Kennedy said. “The koi came right up to you, expecting food. It’s amazing that [the thieves] did this right in front of all of us.”

After the koi were netted and loaded into coolers, they were driven away in an SUV, police said. The men returned the next day, a Sunday, and then again during the next weekend to collect more koi. Each time, they worked between 1 and 4 p.m.

When they were done, only a few fish remained in the pond. The theft was discovered on June 17.

“We’ve never heard of a theft like this before,” said Lucy Caldwell, a Fairfax police spokeswoman.

But there have been others across the nation. In May, eight koi worth about $1,600 were stolen from a pond at the University of Wisconsin campus in Madison. In January, nine koi were stolen from a Florida woman, and in 2010, 24 koi were swiped from a family’s Scarsdale, N.Y., pond.

But the scale of the heist in Fairfax surprised koi aficionados — all said they had never heard of a larger one. Most said they did not believe that there was a black market for the fish but that a thief could easily resell them to a dealer.

“How are you going to know where those koi came from?” said Gray, of the Mid-Atlantic Koi Club. “He could say, ‘I’m emptying my pond.’ They could have already been sold.”