What percentage of inmates at the federal, state, and local level are illegal aliens
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Executive summary
Available federal data show that a large share of federally sentenced non‑U.S. citizens were classified as “illegal aliens” — the U.S. Sentencing Commission reported 88.7% of federally sentenced non‑U.S. citizens were illegal aliens [1]. Other official and watchdog sources show that estimating the share of inmates who are unauthorized at the federal, state, and local levels is complicated by differing data collection systems, changing ICE reporting practices, and point‑in‑time snapshots that undercount flows [2] [3] [4].
1. Why a single percentage doesn’t exist: incompatible data systems
No single, authoritative percentage of “illegal alien” inmates across federal, state, and local systems appears in the available reporting. Federal prison citizen‑status data are maintained by the Bureau of Prisons and reported into Justice Department/BJS products, but the BJS point‑in‑time approach and agency reporting mean numbers reflect year‑end snapshots rather than all people who cycle through custody during a year [2]. State and local jail counts historically relied on DOJ’s SCAAP reimbursements, which changed participation and therefore shifted totals — GAO’s review found the criminal‑alien share in federal prisons fell from about 25% to 21% during 2011–2016 and that SCAAP participation declines reduced state/local counts [4].
2. What we can say about federally sentenced inmates
The U.S. Sentencing Commission provides a clear figure for a defined population: among federally sentenced non‑U.S. citizens, 88.7% were categorized as illegal aliens in a 2025 quick‑facts release [1]. That statistic describes only the subset of people sentenced in federal courts who were non‑U.S. citizens; it does not describe the entire federal inmate population nor state and local systems [1].
3. ICE and “criminal alien” arrest statistics do not equal inmate‑population percentages
ICE and DHS press releases emphasize that a high share of ICE arrests involve people charged with or convicted of crimes (multiple DHS releases claim “70% of ICE arrests are of criminal illegal aliens”), but those statements reflect ICE enforcement activity, not the composition of prison or jail populations nationwide [5] [6]. ICE’s Criminal Alien Program focuses on identifying and removing incarcerated noncitizens from federal, state, and local custody; the program’s activity and publicity campaigns are not equivalent to independent counts of how many inmates in each system are unauthorized [7].
4. Local and state complicating factors: reporting changes and policy choices
Recent analyses show ICE altered how it codes arrests, making distinctions among federal, state and local arrests harder to trace; Prison Policy’s look at ICE arrest data notes ICE stopped distinguishing some program pathways in mid‑2025, which obscures facility‑level sourcing [3]. Additionally, state and local cooperation choices (detainer practices, transfers to ICE) influence how many noncitizen detainees are ultimately moved into federal immigration custody, producing big variation by jurisdiction [3] [8].
5. Historical context from GAO and trends
GAO and earlier DOJ data indicate the criminal‑alien share of federal inmates declined modestly in the 2010s (from roughly 25% to about 21% in the period GAO reviewed) and that SCAAP‑reported incarcerations in state and local facilities fell substantially from 2010 to 2015 — partly due to fewer participating jurisdictions [4]. Those trends show both real changes and measurement artifacts driven by program participation and reporting practices [4].
6. What the available sources do not answer
Available sources do not provide a single, up‑to‑date nationwide percentage that says “X% of all federal, state and local inmates are illegal aliens.” The Sentencing Commission gives a precise percentage for federally sentenced non‑U.S. citizens (88.7%) but does not cover state/local systems [1]. ICE enforcement and DHS press releases emphasize criminal‑alien arrests but do not translate directly into a comprehensive inmate‑population share [5] [6]. GAO and Prison Policy note measurement limits and shifting reporting that prevent a simple aggregation [2] [3].
7. How to get closer to an answer
To estimate system‑wide shares reliably requires combining: BOP/BJS citizenship snapshots for federal prisons (BOP/BJS data described by GAO), comprehensive state corrections reports, and transparent ICE transfer/identification records with consistent coding. GAO’s review and the Prison Policy Initiative recommend improving data harmonization and disclosure to allow accurate cross‑system tallies [2] [3].
Limitations and competing perspectives: federal sentencing data show a concentrated high share of illegal aliens among federally sentenced non‑citizens [1]; DHS/ICE emphasize the criminality of enforcement targets [5] [6]; independent reviewers and auditors warn that reporting practices and jurisdictional differences limit comparability [3] [4] [2].