There are at least three American hostages still being held by the Islamic terrorists who executed journalist James Foley, sources say, along with a small number of others -- perhaps a dozen. But a rescue mission would be very difficult after Wednesday's revelation that the U.S. military made a failed rescue attempt earlier this summer, CBS News Homeland Security correspondent Bob Orr reports.

The commando raid was a wake-up call for ISIS, also known as the Islamic State of Iraq and Syria, because it's now clear the U.S. is willing to put at least some boots on the ground inside Syria for a while.

Officials believe ISIS is adjusting its operations. Sources say it's likely the remaining hostages have been separated and are being moved around. Also, ISIS may limit its use of cell phones and other communications, making the group more difficult to track.

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To defeat ISIS the U.S. will need to take out its leaders, like Abu Bakr al-Baghdadi, Orr said. But timely and detailed intelligence would be needed to enable the U.S. to target a specific location with an airstrike or a ground raid.

A good example comes from 2006, when a U.S. bombing mission killed Abu Musab al-Zarqawi, the leader of al Qaeda in Iraq. That was successful only because the U.S. had robust intelligence and thousands of troops on the ground able to exploit daily intelligence leads.

The U.S. doesn't have anything close to that in Syria. Sources say there is limited intelligence being collected on the ground.

The disclosure of the failed rescue attempt came after Pentagon officials said several news organizations had obtained details of the operation.

The revelation angered some at the Pentagon who said it will make future rescue attempts more difficult, a fear shared on "CBS This Morning" by former CIA Deputy Director Mike Morell, now a CBS News contributor.

"Now they know we will make an attempt to go after hostages, so they can better protect them," Morell said. "And every little detail that is leaked out about this operation gives ISIS information that will help them protect against future operations. So there was a good reason they tried to keep it secret."

This CBS News animation shows how Black Hawk helicopters would have dropped U.S. commandos at an oil refinery in northern Syria where James Foley and other hostages were believed to be held. CBS News

The hostages, including Foley and three others, were thought to be at a camp inside an oil refinery in northern Syria, an area controlled by ISIS, CBS News State Department correspondent Margaret Brennan reports.

Two dozen special operation forces commandos dropped from Black Hawk helicopters outfitted with stealth technology to avoid detection.

With surveillance aircraft overhead, the team swooped in to search for the hostages. They encountered ISIS militants and engaged in a firefight.

But the hostages were gone.

Several ISIS fighters were killed. One American commando was injured, but all the U.S. forces were flown to safety.

U.S. officials admit the cover mission, launched in early July, was based on good but not great intelligence about the location of the hostages.

The U.S. government had been tracking the American hostages for months and was aware of ransom requests from ISIS.

The defense secretary said the mission was launched because the hostages' lives were thought to be in imminent danger.

"It's the responsibly of our government and our leaders to do all we can to take action when we believe there might be a good possibility, a good chance to make a rescue effort successful," Hagel said.

Hagel also issued a stark warning that the terrorist group is "beyond anything we've seen" because they marry violent ideology with sophisticated military strategy and enough money to carry it out.