Fusion GPS bills itself as a corporate research firm, but in many ways it operates with the secrecy of a spy agency. No sign marks its headquarters above a coffee shop in Northwest Washington. Its website consists of two sentences and an email address. Its client list is closely held.

The small firm has been under intense public scrutiny for producing the 35-page document known as the Trump dossier. Senior executives summoned to testify before Congress in October invoked their Fifth Amendment right against self-incrimination, and the firm is resisting a congressional subpoena for bank records that would reveal who has paid for its services.

But hundreds of internal company documents obtained by The Washington Post reveal how Fusion, a firm led by former journalists, has used investigative reporting techniques and media connections to advance the interests of an eclectic range of clients on Wall Street, in Silicon Valley and in the nation’s capital. The firm has played an unseen role in stories that dominated headlines in recent years.

In the years before it produced the dossier, records show, Fusion worked to blunt aggressive reporting on the medical-device company Theranos, which was later found to have problems with its novel blood-testing technology. It was also hired to ward off scrutiny of the nutritional supplement company Herbalife, which ultimately paid $200 million to distributors to settle claims by regulators.