Christopher Steele, a director of London-based Orbis Business Intelligence Ltd., prepared the dossier, the people said. The document alleges that the Kremlin colluded with Mr. Trump’s presidential campaign and claims that Russian officials have compromising evidence of Mr. Trump’s behavior that could be used to blackmail him. Mr. Trump has dismissed the contents of the dossier as false and Russia has denied the claims.

Mr. Steele, 52 years old, is one of two directors of the firm, along with Christopher Burrows, 58.

Mr. Burrows, reached at his home outside London on Wednesday, said he wouldn’t “confirm or deny” that Orbis had produced the report. A neighbor of Mr. Steele’s said Mr. Steele said he would be away for a few days. In previous weeks Mr. Steele has declined repeated requests for interviews through an intermediary, who said the subject was “too hot.”…

The firm relies on a “global network” of experts and business leaders, provides clients with strategic advice, mounts “intelligence-gathering operations” and conducts “complex, often cross-border investigations,” its website says.