Is ISIS growing?

The number of Islamic State in Iraq and Syria (ISIS) militants may have grown since the U.S. began airstrikes against the terror group, according to Senate Intelligence Committee Chairwoman Dianne FeinsteinDianne Emiel FeinsteinOvernight Health Care — Presented by PCMA — FDA issues proposal to limit sales of flavored e-cigs | Trump health chief gets grilling | Divisions emerge over House drug pricing bills | Dems launch investigation into short-term health plans The Hill's Morning Report - Boeing crisis a test for Trump administration Trump faces growing pressure over Boeing safety concerns MORE (D-Calif.).

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Feinstein said Sunday on CNN that "ISIS is essentially a fighting force of 30,000 to 50,000 people."

That number is a dramatic increase from the most recent estimate, released by the CIA in September.

Then, the CIA said that ISIS's size, based on intelligence reports collected from May to August, was an estimated 20,000 to 31,500.

That figure was also a doubling of an earlier CIA estimate over the summer of at least 10,000.

The CIA told PunditFact on Oct. 7 that the growth was attributed to "battlefield successes and the declaration of a caliphate, greater battlefield activity, and additional intelligence."

The CIA said Monday it did not have an updated estimate and stands by its last estimate.

Officials have suggested, however, that ISIS has a successful online propaganda machine that has fueled recruitment, as well as recent "lone wolf" attacks in Canada and New York.

Feinstein's latest estimate, if accurate, could mean ISIS membership has swelled under U.S.-led military action that began in June.

The U.S. first sent troops to Iraq in mid-June, but began bombing in Iraq on Aug. 8., and in Syria on Sept. 23.

Experts and outside groups have previously said they believe ISIS's size to be as high as 100,000, but Feinstein's estimate is the highest from any U.S. official.