My very pro-Bush administration does not believe that the US DOJ, INS, and other groups have detained U.S. Citizens, green-card holders, and non-citizens alike since September 11th. Or, rather, if this has happened, it is in very select few instances.
It seems easy to find info surrounding this problem (New York Times, ACLU website, NYCLU website). I have found it difficult to find the actual numbers (or estimates) of each group that are currently or have been detained.
How many people are detained in detention centers with no "legal" grounds for their being there? How many are US citizens?
According to the American Civil Liberties Union, between 3000 and 5000 mostly Arab people have been detained by the government as "persons of interest" or "enemy combatants". Only a very small handful have been charged with a crime. Most are released, deported, or simply sent on their way out of the country. Those who are eventually released still might face a year or more behind bars without formal charges being filed or access to a lawyer. The ACLU has sent a request under the Freedom of Information Act to obtain the exact numbers of detainees but the government was only very recently forced to comply. The information is not yet available to the public. A request to the Arab American Institute regarding this issue remains open.