The White House is crippling a Senate inquiry into the government's sluggish response to Hurricane Katrina by barring administration officials from answering questions and failing to hand over documents, senators leading the investigation said Tuesday.

In some cases, staff at the White House and other federal agencies have refused to be interviewed by congressional investigators, said the top Republican and Democrat on the Senate Homeland Security and Governmental Affairs Committee. In addition, agency officials won't answer seemingly innocuous questions about times and dates of meetings and telephone calls with the White House, the senators said.

A White House spokesman said the administration is committed to working with separate Senate and House investigations of the Katrina response but wants to protect the confidentiality of presidential advisers.

"No one believes that the government responded adequately," said Sen. Joe Lieberman, D-Conn. "And we can't put that story together if people feel they're under a gag order from the White House."

Sen. Susan Collins of Maine, the committee's Republican chair, said she respects the White House's reluctance to reveal advice to President Bush from his top aides, which is generally covered by executive privilege.

Still, she criticized the dearth of information from agency officials about their contacts with the White House.

"We are entitled to know if someone from the Department of Homeland Security calls someone at the White House during this whole crisis period," Collins said. "So I think the White House has gone too far in restricting basic information about who called whom on what day."

She added, "It is completely inappropriate" for the White House to bar agency officials from talking to the Senate committee.

White House spokesman Trent Duffy said the administration's deputy homeland security adviser, Ken Rapuano, has briefed House and Senate lawmakers on the federal response. A "lessons learned" report from Homeland Security Adviser Frances Fragos Townsend also is expected in coming weeks, Duffy said.

But he defended the administration's decision to prohibit White House staffers or other presidential advisers from testifying before Congress.

"There is a deliberate process, and the White House has always said it wants to cooperate with the committee but preserve any president's ability to get advice from advisers on a confidential basis," Duffy said. "And that's a critical need for any U.S. president and that is continuing to influence how we cooperate with the committees."

Collins and Lieberman sidestepped questions about whether they plan to subpoena the White House to get the information they seek, though Collins said she does not believe subpoenaing the Homeland Security Department is necessary.

The Senate inquiry is scheduled to conclude in March with a report detailing steps the federal government took — and didn't take — to prepare for the Aug. 29 storm.

Investigators have interviewed about 260 witnesses from federal, state and local governments and the private sector. Additionally, the committee has received an estimated 500,000 documents — including e-mails, memos, supply orders and emergency operation plans — outlining Katrina-related communications among all levels of government.

But Lieberman said the Justice and Health and Human Services departments "have essentially ignored our document requests for months" while HHS has refused to allow interviews of its staff. He described the Homeland Security response as "too little, too late."

Collins offered a rosier view of Homeland Security's cooperation, noting that Deputy Secretary Michael Jackson and department chief of staff John Wood were scheduled to talk to investigators later this week.

A special House committee created to review the government's readiness for Katrina is to release its findings by Feb. 15. Although Rep. Tom Davis, R-Va., the panel's chairman, earlier considered subpoenaing the White House, the panel backed away after the Rapuano briefing.

The panel ultimately did subpoena the Pentagon for Katrina documents, but one lawmaker, Rep. Charlie Melancon, D-La., said he believes Defense Secretary Donald H. Rumsfeld has not handed over enough to fully comply with the legal order.

But in a letter to Melancon on Tuesday, Davis said he is satisfied the Pentagon has complied with the subpoena, which yielded "massive mounds of documents," including classified materials from Rumsfeld.