The inspector general initiated the report last year, in part because of public reports of mistreatment of detainees. Most major agencies have inspectors general, who serve as independent watchdogs with periodic reports on internal matters.

A total of 762 illegal immigrants were jailed in the weeks and months after the attacks on the World Trade Center and the Pentagon, as authorities traced tens of thousands of leads and sought to prevent another attack. Most of the 762 immigrants have now been deported, and none have been charged as terrorists.

The Justice Department has sought to maintain the secrecy of the arrests, fighting news organizations' efforts to gain access to deportation proceedings and for disclosure of more information about the detainees. Public information about the arrests has been fragmented; the report offers the most detailed portrait to date of who was held, the delays many faced in being charged or gaining access to a lawyer, and the abuse that some faced in jail.

The report showed, for instance, that nearly three of every four jailed immigrants were from New York City or New Jersey, many were Pakistanis, and most were arrested within three months of Sept. 11.

The report also found that immigrants arrested in New York and housed at the Metropolitan Detention Center in Brooklyn faced "a pattern of physical and verbal abuse" from some guards as well as "unduly harsh" detention policies.

A total of 84 inmates who were held in Brooklyn in terrorism investigations were subjected to highly restrictive, 23-hour "lockdown," the report found. They were limited to one phone call a week, and they were put in handcuffs, leg irons and heavy chains any time they moved outside their cells, according to the report.

And because of a "communication blackout" in the weeks after Sept. 11, families of some inmates in the Brooklyn facility were told their relatives were not housed there.