A public hearing on Britain's role in the Iraq war opened Tuesday with the chairman of the inquiry commission promising a "fair and frank" investigation.

John Chilcot, the five-member commission's chair, said the inquiry would not be a "whitewash" and he would not shy away from being critical in the wide-ranging probe.

"We want to examine the evidence," Chilcot said. "We will approach our task in a way that is thorough, rigorous, fair and frank."

The commission will question dozens of officials over several months -- including military officials and spy agency chiefs. It will also seek evidence from former White House staff members.

Among the most prominent witnesses will be former Prime Minister Tony Blair, who will be questioned on whether he secretly backed US President George Bush's plans for an invasion a year before Parliament authorized military involvement in 2003.

"We will ask them to explain the main decisions and tasks, and their involvement," Chilcot said. "That will give us a clear understanding of how policy developed and was implemented, and what consideration was given to alternative approaches."

The first five weeks of public testimony, which began Tuesday, was to come from senior officials and military officers. Issues such as equipment, personnel, the "key decisions taken and their rationale," and the legal basis for military action would be covered during the first phase of the public hearings.

Private sessions and analysis would follow before a second round of public sessions in mid-2010.

"In some cases, those hearings will be used to invite witnesses to discuss issues in more detail than in earlier evidence, or to pursue further lines of inquiry," Chilcot said.

The inquiry, which was announced by Prime Minister Gordon Brown in June, will cover the entire eight-year period from the buildup to the war to the withdrawal of British troops.

The inquiry won't apportion blame or establish criminal or civil liability -- only offer reprimands and recommendations in hopes mistakes won't be repeated in the future.

Chilcot said some witnesses may be asked to appear again for more detailed sessions, but not until after the general election, which must be held in June at the latest. The report will not be published until at least the end of 2010.