WASHINGTON (AP) — Lawyers for both the Department of Justice and Google will present arguments Thursday and Friday to conclude the biggest antitrust case in a quarter century.

In closing arguments of a Washington, D.C., trial that began last September, regulators will apply the finishing touches to a case alleging Google has turned its search engine into an illegal monopoly that stifles competition and innovation.

Regulators claim that Google competed unfairly when it made lucrative deals with Apple and other companies to automatically lock its search engine into smartphones and web browsers. Meanwhile, Google maintains that consumers use its dominant search engine because it is the best available option.

“Google pays more than $10 billion per year for these privileged positions,’’ argued Kenneth Dintzer, the Justice Department’s lead litigator, last September. “Google’s contracts ensure that rivals cannot match the search quality ad monetization, especially on phones.”