WASHINGTON — Four U.S. senators have asked the Defense Department for a sweeping review of the Pentagon's failures to quickly get troops in Iraq "the best possible equipment," including armored vehicles that protect against some of the most lethal types of roadside bombs.

The request, contained in a letter sent Wednesday to Defense Secretary Robert Gates, comes eight months after a USA Today investigation showed top Pentagon officials repeatedly balked at requests from troops in Iraq for Mine Resistant Ambush Protected vehicles, or MRAPs.

The MRAP's V-shaped hull deflects the force of bomb blasts, and its safety record is stunning. Only one U.S. serviceman has died in an MRAP since the Pentagon committed in May to sending more than 15,000 MRAPs to Iraq. Currently, about 2,400 are being used by troops there. At least 60 percent of U.S. combat deaths have been caused by roadside bombs.

During a House Armed Services subcommittee hearing Wednesday, top congressmen from both parties also voiced their concerns about the failure to provide troops with necessary equipment, particularly MRAPs.

"I think one thing we can all agree on is that the process was too slow and is still too slow," said Rep. Roscoe Bartlett of Maryland, the senior Republican on the subcommittee.

"Frankly, there is a poor track record in this area, " said subcommittee chairman Gene Taylor, D-Miss. "Improved body armor, uparmored Humvees, jammers, and finally fielding of MRAPs has taken entirely too long to get to the troops."

In the letter, Kit Bond, R-Mo., and Democrats Joseph Biden (Del.), Edward Kennedy (Mass.) and Jay Rockefeller (W.Va.) said they "remain concerned" that troops still are not getting the protection they deserve. In particular, they wrote, the Marines moved too slowly on urgent requests from troops for vehicles that could withstand explosively formed penetrators, or EFPs, bombs that can slice through armor.

"We have to make sure what happened with MRAPS never, ever happens again," Biden said in a separate statement.

In May, USA Today reported that the Marines filed an urgent request from Iraq in January 2007 asking for more protection from EFPs. Although MRAPs are safer than armored Humvees, they are still vulnerable to EFP attacks.

Despite that request, the senators wrote, "we are still five or six months from fielding even limited quantities of EFP-protected vehicles."

"We urge you to conduct a thorough and comprehensive evaluation of all of our wartime acquisition processes," the letter said. "If the effort to provide EFP protection is any guide, unfortunately we are still not accelerating our development and fielding efforts adequate to meet the current threat."

Last week, Biden and Bond had called for an investigation into allegations contained in a report by Marine science adviser Franz Gayl. In the report, Gayl wrote that procurement officers needlessly delayed responding to a February 2005 request for 1,169 MRAPs. The 2005 request from troops wasn't approved until more than a year later, in May 2006.

Pentagon auditors are now reviewing Gayl's study and previous audits, said Lt. Col. Brian Maka, a Defense spokesman.