At the 1991 Madrid Conference, Jordan agreed to negotiate a peace treaty sponsored by the US and the Soviet Union. The Israel-Jordan Treaty of Peace was signed on 26 October 1994. In 1997, in retribution for a bombing, Israeli agents entered Jordan using Canadian passports and poisoned Khaled Meshal, a senior Hamas leader living in Jordan. Bowing to intense international pressure, Israel provided an antidote to the poison and released dozens of political prisoners, including Sheikh Ahmed Yassin, after King Hussein threatened to annul the peace treaty.