The failure of the FBI and the CIA to keep track of Tamerlan Dsarnaev in the months preceding the Boston Marathon bombing has prompted criticism that U.S. law enforcement and intelligence officials ignored important warning signs. The case is reminiscent of criticism leveled at counterterrorism officials after Army Maj. Nidal Hasan's shooting rampage at Fort Hood Texas in November 2009 and after the al-Qaida-directed attempt to blow up a civilian airliner on Christmas Day of that year. In both cases, counterterrorism officials subsequently acknowledged that mistakes had been made. Whether authorities missed important evidence of Dsarnaev's intentions, however, is far less clear. Veteran intelligence officers say resource and legal constraints make it very difficult to follow suspicious individuals closely unless their behavior is genuinely alarming.