WASHINGTON (CNN) -- The Senate Judiciary Committee began a hearing Thursday into the dismissals last year of eight U.S. attorneys, with the chairman accusing the Bush administration of being less than truthful about the firings.

"Regrettably, what we've heard from the administration has been a series of shifting explanations, excuses, lack of accountability or even acknowledgment of the seriousness of this matter," said chairman Sen. Patrick Leahy, D-Vermont.

Leahy said information given to Congress has been contradicted by the sworn testimony of Justice Department attorneys.

Kyle Sampson, former aide to Attorney General Alberto Gonzales, will testify before the committee after the members' opening statements.

Sampson was Gonzales' chief of staff until mid-March, when he resigned over the firings.

Gonzales has said his former aide directed the evaluation process that led to the firings and that his failure to share information about the decisions resulted in Congress getting "incomplete" explanations.