Washington (CNN) Six Democratic senators sent a letter to FBI Director Christopher Wray asking for the names of all White House staff members with interim security clearances, as well as those with completed background checks who are unable to get full clearances.

"You also testified yesterday that the FBI completed a background investigation into White House Staff Secretary Rob Porter in late July, yet he remained in his position until last week," the senators wrote in the letter. "Who else is currently working at the White House following the completion of a background investigation without being able to obtain a permanent security clearance?"

Thirty to 40 White House officials and administration political appointees are still operating without full security clearances, including senior adviser to President Donald Trump Jared Kushner and -- until recently -- White House staffer Rob Porter, according to a US official and a source familiar with the situation.

The White House claims that the backlog of interim security clearances is a procedural consequence of the review process carried out by the FBI and White House Office of Security, which can take time to complete.

Several sources, including intelligence officials who have served previous Democratic and GOP administrations, describe the backlog as very unusual, however, and make clear that the process should have been completed after a year in office.

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