Federal authorities have cracked the case of an international hacker who broke into roughly 100 unclassified U.S. military networks over the past year, officials said Monday.

Officials declined to identify the hacker, a British citizen, but said he could be indicted as early as Tuesday in federal courts in northern Virginia and New Jersey. Those U.S. court jurisdictions include the Pentagon in Virginia and Picatiny Arsenal in New Jersey, one of the Army’s premier research facilities.

The officials declined Monday to say whether this person was already in custody, but one familiar with the investigation, who spoke only on condition of anonymity, said investigators consider the break-ins the work of a professional rather than a recreational hacker.