Walter Shawn Browne, an aggressive, animated chess grandmaster and six-time United States champion who dazzled spectators with his world-class ability at speed chess, died on Wednesday in Las Vegas, where he had been competing in the 50th National Open. He was 66.

He died in his sleep while resting at the home of a friend, according to an announcement on the Las Vegas International Chess Festival’s website. The cause was not yet known. He had tied for ninth in the event, which had concluded on Sunday, held at the Westgate Las Vegas Resort & Casino.

At the board, Mr. Browne was all concentrated energy — scowling, grimacing and grasping his head as he calculated long and complicated variations of moves. Spectators often hovered around his games, attracted by his habit of consuming vast amounts of his allotted time in the early going and then, risking forfeiture, making a flurry of 20 or so moves with only a minute or two left on the clock.

His attacking style of play was especially suited for blitz chess, in which players have to make all their moves in five minutes or less. For decades, Mr. Browne was among the best blitz players in the world and perhaps the best in the United States. In 1988, he founded the World Blitz Chess Association and ran it until it dissolved in 2003.