The unexpectedly swift victory in Fallujah by the American-backed Iraqi military is prompting U.S. officials to ask new questions about the true size of the Islamic State group's fighting force.
After the five-week battle in Fallujah ended Sunday, Iraqi estimates suggest the Islamic State group suffered more than 1,000 casualties and the Iraqi military detained another 1,000 extremists, said Army Col. Chris Garver.
That will be hard to replenish. The estimated flow of foreign fighters heading into Iraq and Syria to join the ranks of the Islamic State group has dropped from more than 2,000 fighters a month last year to current estimates as low as 200 per month, Garver said.
Officially, the U.S. intelligence community estimates the number of Islamic State militants at somewhere between 19,000 and 25,000, a figure last updated in February.
Now, as the focus of the counter-Islamic State effort shifts toward the militants' stronghold in Mosul, U.S. military officials in Baghdad are drawing up some new estimates for the size of the Islamic State force, also known as ISIS or ISIL.
"We're definitely trying to figure that out," Garver, the top spokesman for the U.S. headquarters in Iraq, said Wednesday in a briefing with reporters.
"I have not seen another estimate that addresses that after the success we've seen, the progress that the Iraq security forces made in Fallujah," Garver said.
Shortly after the four-month battle for Ramadi ended in February, U.S. intelligence experts ratcheted back their official estimate for the size of ISIS from a range of 25,000 to 30,000 to the current estimate as low as 19,000.
There is some evidence that ISIS no longer views fighting in Iraq and Syria to be its primary mission. ISIS-controlled media have stopped urging sympathizers to travel to the self-proclaimed caliphate. "They're saying, 'Hey, don't come to Syria anymore. Do an attack at home," said Brett McGurk, the White House's special envoy for the counter-ISIS fight, at a Senate hearing Tuesday.
Before the Iraqis invaded Fallujah in May, many military experts wondered how ISIS would respond, whether the militants would stand and fight or flee.
Initial analysis of the battle shows signs of both.
In Fallujah, the Islamic State group "was not monolithic," Garver said.
"Some people fought harder than other people did. Some people tried to melt away," Garver said. In "different neighborhoods, you get a different answer."
The Fallujah battle may suggest ISIS' fighting power has diminished.
"The resistance wasn't as stiff in Fallujah as what we saw in Ramadi.
"As for why that is? ... We're going to have to try to figure that out through the intelligence process to be able to determine why that is," Garver said.