Ballistic tests on 103 vests containing a fiber known as Zylon produced acceptable safety results for just four vests, department researchers said.
"This confirms that these vests simply don't do what they claim to do, which is to stop bullets," said Ed Balzarini Jr., a lawyer from the Pittsburgh metropolitan region who represents an officer in Forest Hills, Pa., Ed Limbacher, who was seriously wounded in 2003 in drug raid when a bullet pierced his vest and lodged in his abdomen.
The shooting death of another officer, in Oceanside, Calif., was linked to a similar type vest.
Police armor using Zylon, patented by a Japanese company, became popular about a decade ago as a lighter alternative to hotter, bulkier vests. The material is found in more than 240,000 vests bought by police departments in the United States in recent years, officials said.
Many departments have stopped using Zylon vests in the last two years in light of increased safety concerns and a flurry of lawsuits against manufacturers.
But law enforcement officials said tens of thousands of officers continued to rely on them.
As a result of its findings, the Justice Department said it would commit $33.6 million to help police departments replace Zylon vests. It also imposed new safety standards for Zylon vests and said it would no longer allow federal reimbursement for departments that bought them.
In New York City, Deputy Commissioner Paul J. Browne, chief spokesman for the Police Department, said the department did not use Zylon vests.
"All bullet-resistant vests issued and inspected annually by the New York City Police Department are made of Kevlar," Commissioner Browne said, referring to another material that federal officials said had shown no sign of failures. "None are made of Zylon. We have no known failures. In fact, the current models in use were instrumental in stopping multiple gunshots from penetrating officers' vests in several close-quarters gunfights this summer."
The Justice Department, which began studying Zylon vests in 2003, found in earlier tests that the material deteriorated quickly, particularly when exposed to light, heat and moisture. The findings released on Wednesday were the first definitive tally of the failure rate, law enforcement officials said.
"We expected the Justice Department to find some level of deficiency in these vests, but this level is startling," said Jim Pasco, executive director of the Fraternal Order of Police, the law enforcement association that first alerted federal officials to potential dangers in Zylon vests.
"We're obviously concerned by these results," Mr. Pasco said in an interview, "but, thank God, we're finding this out now rather than later, because this is a critical issue for officer safety, and the adverse effects could be horrible otherwise."
The tests by the National Institute of Justice, an arm of the Justice Department, used 9-millimeter, .357 Magnum and other ammunition on 103 Zylon vests from law enforcement agencies around the country. In 60 cases, or 58 percent, at least one bullet from a six-shot series penetrated the vest. Even in those cases where the bullet did not pierce the armor, 91 percent of the vests sustained damage considered excessive enough to cause blunt-trauma injuries to the officers wearing them, the researchers said.
"We think this is an unacceptable risk of serious bodily injury to officers," Sarah V. Hart, director of the justice institute, said in an interview, "and these vests should be replaced."
The Justice Department emphasized in its report that until police departments and officers could replace their vests, even armor with Zylon was "better than no armor."
A spokesman for Armor Holdings of Jacksonville, Fla., widely considered the largest manufacturer of police vests in the United States, said Wednesday that the company continued to believe that Zylon vests were safe.
The spokesman, Michael Fox, said his company, which makes several types of vests with Zylon, was studying the new findings and welcomed the new standards.
"We're confident in the performance of our products," Mr. Fox said. "These products have been in the field for a long time. They have saved dozens and dozens of lives, and they're working."