Christopher Steele alerted FBI to dossier because he worried Trump was being blackmailed
Donald Trump speaks at the Republican National Convention in Cleveland, Ohio, on July 18, 2016 (AFP Photo/Robyn Beck)

A former British spy handed over his research dossier to the FBI in July 2016 because he feared the Republican Party was about to nominate a candidate who was being blackmailed.

Christopher Steele had been hired by opposition research firm Fusion GPS to look into Donald Trump's background, and what he found so alarmed him that he alerted U.S. law enforcement.

"He thought from his perspective there was an issue -- a security issue about whether a presidential candidate was being blackmailed," said Glenn Simpson, head of Fusion GPS.

Simpson testified before the Senate Judiciary Committee on Aug. 22, but Democrats and Republicans have wrangled over whether transcripts should be released.

Fusion GPS and its lawyers have called for their release, but the GOP majority resisted -- until Sen. Dianne Feinstein (D-CA) leaked the lengthy transcript Tuesday.

Republicans have claimed the dossier, which was partially funded by Hillary Clinton's campaign and the Democratic National Committee, formed the basis of an FBI investigation launched in July 2016.

But the New York Times reported earlier this month the investigation was prompted after campaign adviser George Papadopolous boasted over wine to an Australian ambassador about his contacts with the Russian government.