The two victims were couriers from Hong Kong who were in the Seattle area to sell the cache of diamonds, according to the King County Sheriff’s Office.

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A brazen group of robbers boxed in a diamond couriers’ car on an Interstate 5 onramp, held the driver at gunpoint and stole $400,000 worth of gems in SeaTac Tuesday, authorities said.

The two victims, both men, were couriers in town from Hong Kong to sell the cache of diamonds, according to the King County Sheriff’s Office.

They were getting onto northbound I-5 about noon when their car was cut off by a second vehicle, the sheriff’s office said. As the couriers’ car stopped, another vehicle slammed into it from behind, pinning them in.

A witness told deputies one person pointed a handgun at the couriers, while two others wearing masks and sunglasses approached their car and broke its windows, the sheriff’s office said. One person then reached inside and grabbed a backpack containing the diamonds.

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Before fleeing, the robbers punctured tires on the couriers’ car.

There is no indication that the couriers and the robbers knew each other, King County sheriff’s Sgt. Cindi West said. Investigators believe the diamond couriers came to the area to do business and somehow, “Their information was broadcasted,” she said.

“Somebody knew they were coming.”

There is also no indication that the diamonds were here illegally, she said. When asked where the couriers were headed, or with whom they were doing business, she said investigators would not release that information.

Sheriff’s detectives have been in contact with Bellevue and Kirkland police, who have investigated similar cases in recent years, West said. Investigators are trying to determine if those incidents are connected with Tuesday’s robbery. Spokespeople for those police departments could not immediately be reached.

Investigators are asking for the public’s help in identifying the robbers. West said other vehicles were in the area at the time, and those drivers may have information.

The robbers’ vehicle is described as a black, four-door sedan, possibly a BMW or Mercedes-Benz, the department said. Witnesses described the robbers as wearing black clothing, including hooded jackets, masks and sunglasses.

There was no description of the vehicle used to ram the couriers’ car.

Jewelry heists aren’t unheard of in the Seattle area.

In April 2013, four men robbed a traveling jewelry courier as he was getting out of his car at Northgate Mall. The masked men, armed with ice picks, pulled up in a van and jumped out, pushed the courier backward and punctured the rear tires on his car.

One month earlier, $500,000 in jewels was stolen from a courier’s vehicle as he was eating lunch at a deli on Mercer Island. The New York-based courier was in the area showing diamond wedding rings to stores and wholesalers, Mercer Island police said.

In one of the region’s most brazen heists, a Friedlander’s Jewelers district manager and security guards were jumped by a group of men as they brought a case of loose diamonds from Southcenter Mall to Bellevue Square on Oct. 18, 1996. The victims were forced to the ground and the robbers fled with the diamonds after flattening the tires of witnesses.

Authorities are asking anyone with information about the SeaTac robbery to call the King County Sheriff’s Office at 206-296-3311 or CrimeStoppers at 1-800-222-TIPS.