Google acted illegally to maintain a monopoly in online search, a US judge ruled on Monday, a decision that strikes at the power of big tech companies and that may fundamentally alter the way they do business. Judge Amit Mehta of US District Court for the District of Columbia said Google had abused a monopoly over the search business. The Justice Department and states had sued Google, accusing it of illegally cementing its dominance, in part, by paying other companies, like Apple and Samsung, billions of dollars a year to have Google automatically handle search queries on their smartphones and web browsers.