Washington ? Bullets fired in government tests penetrated more than half the police body armor vests containing the synthetic fiber Zylon, already the subject of lawsuits over reliability.

As a result, the government announced that it would no longer help local police forces pay for bullet-resistant vests that contain any Zylon but would add $10 million to the $23 million already available for police vests.

The Justice Department tests showed that vests made with Zylon lose strength over time, well before their standard five-year warranty expires and even when the armor appears to be in good condition, according to the study released Wednesday by the department’s National Institute of Justice.

The prime factor leading to Zylon’s degradation appears to be moisture, the study said, adding that more research is needed. Keeping protective material dry is an especially tricky problem – officers often perspire and sometimes work in rainy or snowy conditions.