The case for manipulation is pretty strong. It relies on several things,
but I think the most important of them has been the discovery that the
administration deliberately suppressed dissenting views on some of the most
important pieces of evidence that they used to bolster their case for war. For
future reference, here's a list of five key dissents about administration
claims, all of which were circulated before the war but kept under wraps until
after the war:

The Claim: Ibn al-Shaykh al-Libi, an al-Qaeda prisoner captured in
2001, was the source of intelligence that Saddam Hussein had trained al-Qaeda
members to use biological and chemical weapons. This information was used
extensively by Colin Powell in his February 2003 speech to the UN.

What We Know Now: As early as February 2002, the Defense Intelligence
Agency circulated a report, labeled DITSUM No. 044-02, saying that it was
"likely this individual is intentionally misleading the debriefers." Link.
This assessment was hidden from the public until after the war.

The Claim: An Iraqi defector codenamed "Curveball" was the source of
reporting that Saddam Hussein had built a fleet of mobile biowarfare labs.
Curveball's claims of mobile bio labs were repeated by many administration
figures during the runup to war.

What We Know Now: The only American agent to actually meet with Curveball
before the war warned that he appeared to be an alcoholic and was unreliable.
However, his superior in the CIA told him it was best to keep quiet about this:
"Let's keep in mind the fact that this war's going to happen regardless of what
Curveball said or didn't say, and the powers that be probably aren't terribly
interested in whether Curveball knows what he's talking about." Link.
This dissent was not made public until 2004, in a response to the SSCI
report that was written by Senator Dianne Feinstein. Link.

The Claim: Iraq had purchased thousands of aluminum tubes to act as
centrifuges for the creation of bomb grade uranium. Dick Cheney said they were
"irrefutable evidence" of an Iraqi nuclear program and George Bush cited them in
his 2003 State of the Union address.

What We Know Now: Centrifuge experts at the Oak Ridge Office of the
Department of Energy had concluded long before the war that the tubes were
unsuitable for centrifuge work and were probably meant for use in artillery
rockets. The State Department concurred. Link. Both of these dissents were omitted from the CIA's declassified National Intelligence Estimate, released on October 4, 2002. Link. They were subsequently made public after the war, on July 18, 2003. Link.

The Claim: Saddam Hussein attempted to purchase uranium yellowcake from
Africa as part of his attempt to reconstitute his nuclear program. President
Bush cited this publicly in his 2003 State of the Union address.

What We Know Now: The primary piece of evidence for this claim was a
document showing that Iraq had signed a contract to buy yellowcake from Niger.
However, the CIA specifically told the White House in October 2002 that the
"reporting was weak" and that they disagreed with the British about the
reliability of this intelligence. Link. At the same time, the State Department wrote that the documents were "completely implausible." Link.

Three months later, in January 2003, Alan Foley, head of the CIA's
counterproliferation effort, tried to persuade the White House not to include
the claim in the SOTU because the information wasn't solid enough, but was
overruled. Link. Five weeks later, the documents were conclusively shown to be forgeries. Link. In July 2003, after the war had ended, CIA Director George Tenet admitted publicly that that the claim should never have been made. Link.

The Claim: Saddam Hussein was developing long range aerial drones capable
of attacking the continental United States with chemical or biological weapons.
President Bush made this claim in a speech in October 2002 and Colin Powell
repeated it during his speech to the UN in February 2003.

What We Know Now: The Iraqi drones had nowhere near the range to reach the
United States, and Air Force experts also doubted that they were designed to
deliver WMD. However, their dissent was left out of the October 2002 NIE and
wasn't made public until July 2003. Link.