The FBI agent who arrested Zacarias Moussaoui in August 2001 testified Monday he spent almost four weeks trying to warn U.S. officials about the radical Islamic student pilot but “criminal negligence” by superiors in Washington thwarted a chance to stop the Sept. 11 attacks.

FBI agent Harry Samit of Minneapolis originally testified as a government witness, on March 9, but his daylong cross examination by defense attorney Edward MacMahon was the strongest moment so far for the court-appointed lawyers defending Moussaoui. The 37-year-old Frenchman of Moroccan descent is the only person charged in this country in connection with al-Qaida’s Sept. 11, 2001, attacks on the World Trade Center and the Pentagon.