(CNN) A pair of Republican staffers on the House intelligence committee flew to London recently to try to find a way to bring the author of a dossier containing explosive allegations about President Donald Trump and top members of his campaign before the panel, sources familiar with the trip have told CNN.

A congressional source familiar with the trip said House investigators are expected to send a letter to the author of the file, Christopher Steele, inviting him to testify before the House intelligence committee -- but it will have to be approved by the leaders of the panel's investigation first.

Steele, a former British intelligence official, has been a highly sought-after witness ever since it was revealed he wrote the dossier, which contained unverified claims that Russian operatives had compromising personal and financial information about Trump.

Trump always denied the claims, dismissing them as "phony" in January.

The trip by House Republican staff investigating Russian interference in the US election touched off a spat behind the scenes at the Capitol because Democrats on the investigation were kept in the dark, according to a separate source familiar with the trip.

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