After two days of investigation into the mass shooting at Fort Hood, investigators have tentatively concluded that the attack was not part of a terrorist plot. Rather, they have come to believe that Maj. Nidal Malik Hasan, the Army psychiatrist accused in the shootings, acted out under a welter of emotional, ideological and religious pressures, according to interviews with federal officials who have been briefed on the inquiry. Investigators have not ruled out the possibility that Major Hasan believed he was carrying out an extremist’s suicide mission. But the investigators, working with behavioral experts, suggested that he may have long...