Seattle-based Double Down Interactive, developer of casino-style online social games, is being purchased by Nevada-based slot machine maker International Game Technology for $500 million.

The sale marks another blockbuster deal involving a Seattle game development company. California-based Electronic Arts (EA) acquired PopCap Games last year in a deal valued at up to $1.3 billion.

With the Double Down deal, IGT is getting a fast-growing company behind the popular DoubleDown Casino game on Facebook, which has 4.7 million monthly active users, up from 3.3 million in October.

Double Down will remain in Seattle, according to IGT's announcement of the deal.

International Game Technology is based in Reno and makes slot machines and video poker games. IGT said by buying Double Down, it gains “instant size and scale in the fast-growing world of casino-style social gaming and is expected to broaden IGT’s popular gaming titles beyond the physical casino to Facebook.”

The deal has intrigue, given the fact that IGT supplies casino and slot games to sites outside the U.S. where online gambling is legal. Double Down offers blackjack, slots, slot tournaments, video poker and roulette to social gamers for fun without any cash jackpots.

But could that change one day? It's hard to say.

Online gambling is currently illegal in the U.S. But there is pressure to change the law and legalize online poker, lotteries and other forms of internet gambling. The U.S. Department of Justice recently said it was it reversing its interpretation of the federal 1961 Wire Act, potentially opening the door for legalized online gambling in the U.S.

To get Double Down, IGT plans to pay $250 million in cash, $85 million in retention payments over the next two years and up to $165 million in cash payable over the next three years if Double Down meets financial performance targets.

Double Down, which started in 2008, has been adding workers and recently announced that it hired former Real Networks executive Charles Harper as vice president of business development. Greg Enell, CEO and co-founder, will continue to lead Double Down after the sale.

“Double Down and IGT share complementary cultures focused on innovation and creativity. Both companies are committed to providing unrivaled quality, service and entertainment to millions of players,” said Enell. “Leveraging IGT’s Research and Development, global reach, and best-in-class content will provide our loyal player base with an even more robust experience and is expected to augment Double Down’s growth trajectory.”

In September, the company announced it was relocating to a new space, 505 Union Station, near Seattle’s historic Pioneer Square. The company has kept up a brisk hiring pace. Double Down Interactive has 70 full-time employees and openings for than 10 positions.