US authorities knew that an army major accused of killing 13 people at a military base had been in contact with a cleric sympathetic to al-Qaeda.
Intelligence agencies monitoring the e-mail of Yemen-based Anwar al-Awlaki said he had communicated with Maj Nidal Malik Hasan on 10 to 20 occasions.
However, it was decided that this did not merit further investigation.
US officials say Maj Hasan apparently acted alone in carrying out Thursday’s massacre at Fort Hood base in Texas.
President Barack Obama is due to visit Fort Hood later on Tuesday for a memorial service for the 13 people who died in last week’s shootings.