LONDON - The first full-scale inquiry into Britain's role in the Iraq war opened Tuesday with families of soldiers killed in combat desperate to hear Tony Blair justify the decision to join the US-led invasion.

More than six years after the then prime minister took Britain to war, inquiry chairman John Chilcot said no-one was "on trial" in the probe, which is due to last a year, but promised not to shy away from criticism.

The first day of hearings was dominated by testimony by top civil servants who told how long-standing concerns about Saddam Hussein's regime hardened after the September 11, 2001, attacks in New York and Washington.

"No-one is on trial here. We cannot determine guilt or innocence. Only a court can do that," Chilcot said in his opening remarks at a conference centre in central London.

"But I make a commitment here that once we get to our final report, we will not shy away from making criticisms, either of institutions or processes or individuals, where they are truly warranted."

The committee has already met with families of some of the 179 British troops who died in Iraq, a number of whom also attended Tuesday's session.

"I just want the truth," Rose Gentle, whose son Gordon died in Iraq in 2004, told AFP afterwards. She said it was early days but she had had confidence in some of the committee members to probe deep.

"I've never had any answers. I've never been told anything. Why we went in, whether it was legal," she added.

Gentle, who wears a picture of her son in a gold heart around her neck, said she would return when Blair gives evidence. "If mistakes were made, he's the one that's got to live with it," she said.

An appearance by Blair, pencilled in for January, is likely to be the highlight of the hearings.

A small group of protesters gathered outside the inquiry venue, wearing masks of Blair, former US president George W. Bush and current British Prime Minister Gordon Brown, and with fake blood on their hands.

Anti-war campaigners want a ruling on the legality of the conflict, which took place without explicit approval from the UN Security Council.

Inside, there seemed to be little public interest, by contrast to the million Britons who marched against the invasion on one day in 2003 -- only about half of the seats in the public gallery were filled.

In the first session Tuesday, senior British civil servants said Iraq was considered a threat in 2001 because of a "clear impression" that it intended to "acquire WMD (weapons of mass destruction) capability".

Iraq's suspected possession of such weapons was the main justification for the invasion in March 2003, but they were never found.

There were "voices" in Washington in 2001 talking about deposing Hussein, but US and British policy was focused on containing the Iraqi leader's ambitions through sanctions and a no-fly zone, the officials said.

William Patey, head of the Middle East department at the Foreign Office in 2001, said he ordered a memo in late 2001 detailing "all the options" for Iraq. It included regime change, but he said this was quickly dismissed.

Peter Ricketts, who chaired the government's top intelligence committee in 2000-2001, added: "I was certainly not aware of anyone in the British government promoting or supporting active measures for regime change."

Thinking in Washington and London shifted after the September 11 attacks, the hearing heard, and concern moved to what terrorists might do if they got hold of biological, chemical or nuclear weapons.

"The shift in thinking was to say that we cannot afford to wait for these threats to materialise. We must be ready to engage potential threats wherever they emerge," said Simon Webb, then policy director at the Ministry of Defence.

The inquiry is looking at all elements of British involvement in Iraq between 2001 and 2009 when nearly all British troops pulled out.

There have already been two official probes into elements surrounding the run-up to the invasion, but ministers had refused to hold a full inquiry until after the military deployment had ended.