Washington (CNN) 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.

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

Asked on Thursday why so many White House officials were still operating on interim clearances, White House spokesman Raj Shah said he wasn't able to discuss specifics of the clearance process.

One current and one former US official said the backlog could indicate that there are remaining questions or obstacles from the intelligence community and law enforcement conducting the review.

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