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Colorado officials are offering a reward of up to $100,000 for information related to the decades-old cold case double murder of teenage "high school sweethearts" at a Subway restaurant.

Metro Denver Crime Stoppers announced the six-figure reward on Sunday – the 21st anniversary of the slayings – as investigators push forward with efforts to find the person or people who gunned down Nicholas "Nick" Kunselman and Stephanie Hart-Grizzell in Littleton, Colo., just blocks from Columbine High School.

The reward was increased from the previously offered $12,000, local affiliate FOX 31 reported. 

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"The reward in this case is now up to $100,000. What this shows is people still care, people want answers, people want justice for Nick and Stephanie and the entire community," Metro Denver Crime Stoppers' Sarah Johnston told the news station.

According to a post on the Crime Stoppers Facebook page, a Subway employee called police just after 12:45 a.m. on Feb. 14, 2000, after passing the restaurant on West Coal Mine Avenue and noticing that the store’s interior lights had been left on.

The employee went inside to find Hart-Grizzell, 16, and 15-year-old Kunselman fatally wounded behind the counter, the post states.

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Officials said Kunselman was working at the store, while Hart-Grizzell had been waiting for him to finish his shift, when one or more intruders entered and, at some point, opened fire.

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"With this significantly increased reward, people who haven’t come forward with their information will be much more likely to do so now," said Jefferson County Sheriff Jeff Shrader. "We are continuing our diligent work to bring justice to Nicholas and Stephanie."

Anyone who believes they have information in connection with the case can call 720-913-7867 or submit a tip online by clicking here.