BERLIN, Feb. 26 -- A United Nations special investigator has concluded in a report scheduled for release Friday that foreign intelligence agents sent to question U.S.-held terrorism suspects at Guantanamo Bay had violated international human-rights laws.

According to an advance copy of the report, obtained by The Washington Post, Martin Scheinin, a Finnish diplomat and the U.N. special investigator for human rights, said foreign agents visiting Guantanamo or secret U.S. jails overseas committed "an internationally wrongful act" even if they merely observed interrogations...