The waves of cyber-attacks that crippled government and commercial websites in the US and South Korea relied on roughly 200,000 computers, more than triple earlier estimates, and used an established technique for spreading the infections through other compromised internet pages, researchers said.

US investigators also found that the computers, which were used as pawns in the assaults that started over the weekend, were checking for further instructions from two master computers in Germany and one in Austria, which authorities had disconnected. The identification of those command servers provided some hope that the probe would lead to individual suspects or groups.