This situation is more common than most people realize and creates genuine confusion because the search tools that work for federal inmates do not always work when a federal detainee is being held locally.
The Bureau of Prisons inmate locator at bop.gov is the authoritative database for federal inmates in BOP custody. If your person shows up there, you have your answer with facility and contact information. But if they do not appear in the BOP system, it does not mean they are not a federal inmate. It means they are not yet in BOP custody.
Federal defendants who are awaiting trial, waiting for sentencing, or waiting for a facility designation after sentencing are frequently housed in county or city jails under contract with the U.S. Marshals Service. The Marshals use local facilities to hold federal detainees because the federal system does not have enough detention space to house everyone at the pretrial stage. During that period the person is technically a federal inmate but physically sitting in a local jail that does not appear in the BOP database.
For finding someone in that situation, VINELink at vinelink.com is the most useful tool. It pulls custody data from local jails and county facilities across the country and can surface someone who would not appear in a federal search. Search by full legal name and work through the state where you believe they are being held.
If VINELink does not return results, calling the U.S. Marshals Service district office in the jurisdiction where the case is being prosecuted is another avenue. They track where federal detainees are being held even when those detainees are in local facilities.