WASHINGTON (AP) _ The Justice Department launched an intensive review Tuesday of the reliability of police body armor, which officials say can lose strength over time and put officers' lives in danger.

The initiative will focus on vests made with the bullet-resistant material Zylon, manufactured by Toyobo Co. Ltd. in Japan and used in many types of body armor.

Chuck Canterbury, president of the Fraternal Order of Police, said in a letter Monday to Attorney General John Ashcroft that the company has acknowledged Zylon may lose 20 percent of its strength within just two years. The vests carry a five-year warranty.

Ashcroft said in a statement that the review is intended to ``ensure the reliability of bullet-resistant vests worn by officers as they patrol our streets and keep our communities safe.

``As all law enforcement organizations know, bulletproof vests exist to save the lives of law enforcement officers,'' Ashcroft said.

Vests have saved an estimated 2,700 officers' lives over the past 30 years, Ashcroft said. FBI statistics also show that 324 officers who were wearing body armor have been fatally shot over the past decade, 120 of those in the upper torso that is protected by the vest.

In 2002, one officer was fatally shot through his vest, with a high-powered round from a .30-.30 rifle, according to the FBI.

Questions about the vests have led some police executives to warn their officers against wearing them, which Canterbury said actually makes things more dangerous. If the Justice Department finds that Zylon does degrade quickly, the FOP wants the government to stop certifying the vests and force their makers to replace them.

Announcement of the review comes one day after the Massachusetts attorney general filed a lawsuit seeking to stop sale of Zylon-based vests in that state.

Officials at Toyobo said they welcomed the Justice Department review and they had turned over all of their internal findings about Zylon to the government. Those documents indicate that the vest's design and weave pattern can play a role in deterioration of Zylon and that not all vest makers experienced the same problems.

``We think this is a positive development,'' said company spokesman Kent Jarrell. ``This is where the investigation belongs.''

The initiative announced by Ashcroft will include:

_A study by the National Institute of Justice on Zylon-based vests and how they are certified by the government.

_A summit of law enforcement organizations, vest makers and testing groups to go over the study and determine whether Zylon-based armor remains suitable for police.

_Assistance for state and local law enforcement agencies in replacing any defective equipment.

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On the Net:

Justice Department: http://www.usdoj.gov

Fraternal Order of Police: http://www.grandlodgefop.org