Analysts at U.S. Central Command were pressured to ease off negative assessments about the Islamic State threat and were even told in an email to “cut it out,” Fox News has learned – as an investigation expands into whether intelligence reports were altered to present a more positive picture.

Fox News is told by a source close to the CENTCOM analysts that the pressure on them included at least two emails saying they needed to “cut it out” and “toe the line.”

Separately, a former Pentagon official told Fox News there apparently was an attempt to destroy the communications. The Pentagon official said the email warnings were "not well received" by the analysts.

Those emails, among others, are now in the possession of the Pentagon inspector general. The IG’s probe is expanding into whether intelligence assessments were changed to give a more positive picture of the anti-ISIS campaign.

The former Pentagon official said there were “multiple assessments” from military intelligence and the CIA regarding the “rapid rise” of ISIS in Iraq and North Africa in the year leading up to the group’s territory grab in 2014.

Similar intelligence was included in the President’s Daily Brief, or PDB – the intelligence community’s most authoritative product -- during the same time period. Yet the official, who was part of the White House discussions, said the administration kept "kicking the can down the road." The official said there was no discussion of the military involvement needed to make a difference.

The IG probe started earlier this year amid complaints that information was changed to make ISIS look more degraded than it really was.

Among the complaints is that after the U.S. air campaign started in August 2014, the metrics to measure progress changed. They were modified to use measures such as the number of sorties and body counts -- a metric not used since the Vietnam War -- to paint a more positive picture.

Critics say this "activity-based approach" to tracking the effectiveness of strikes does not paint a comprehensive picture of whether ISIS is being degraded and contained.

The New York Times first reported on Sunday that the IG investigation was expanding and adding more investigators, and that the office had taken possession of a trove of documents and emails as part of that probe.

Asked about the report, House intelligence committee Chairman Devin Nunes, R-Calif., said Sunday that his committee and others are involved in the investigation.

“We heard from a lot of whistle-blowers and other informants who have given us information. And not just … related strictly to the latest allegations,” Nunes said on CNN’s “State of the Union.”

Citing the renewed focus on ISIS after the Paris terror attacks, he added: “So the president, to have a successful strategy, is going to admit that they've got it wrong and they need to relook at a larger strategy that deals with north Africa, the Middle East, all the way over to Afghanistan, Pakistan, and then work closely with our NATO allies with what appears to be a command and control structure that ISIS has created successfully in Europe.”

President Obama, speaking at a press conference in Malaysia over the weekend, said he expects to “get to the bottom” of whether ISIS intelligence reports were altered – and has told his top military officials as much.

“One of the things I insisted on the day I walked into the Oval Office was that I don’t want intelligence shaded by politics. I don’t want it shaded by the desire to tell a feel-good story,” Obama said Sunday. “I believe that the Department of Defense and all those who head up our intelligence agencies understand that, and that I have made it repeatedly clear to all my top national security advisers that I never want them to hold back, even if the intelligence or their opinions about the intelligence, their analysis or interpretations of the data contradict current policy.”

At the same time, he said, “As a consumer of this intelligence, it’s not as if I’ve been receiving wonderfully rosy, glowing portraits of what’s been happening in Iraq and Syria over the last year and a half. … [I]t feels to me like, at my level at least, we’ve had a pretty clear-eyed, sober assessment.”

The president’s call for a thorough investigation was greeted with cynicism by those involved in the 2014 intelligence assessments, since the administration did not act on the earlier raw intelligence that painted a dire picture of developments, especially in Iraq.