Maj Hasan was noticed by the FBI in December 2008
The FBI says that a US Army major suspected of killing 13 people was not part of a "broader terrorist plot".
Maj Nidal Hasan was noticed by the FBI in December as part of an unrelated inquiry by the Joint Terrorism Task Force, but did not raise concerns.
Investigators said his communications with another person were in line with his job as an army psychiatrist.
Maj Hasan remains in hospital but has regained consciousness after being shot by police during the attack last week.
Thirteen people died and another 29 were injured in the shooting at Fort Hood base in Texas on Thursday.
The FBI said it had "no information to indicate that Maj Hasan had any co-conspirators or was part of a broader terrorist plot".
The bureau did not name the person Maj Hasan communicated with, and did not confirm reports that he was a radical cleric living in Yemen.
The content of the communications between Maj Hasan and the other individual was assessed as "consistent with research being conducted by Major Hasan in his position as a psychiatrist at the Walter Reed Medical Center" in Washington DC.
'No red flag'
"Because the content of the communications was explainable by his research and nothing else derogatory was found, the JTTF concluded that Major Hasan was not involved in terrorist activities or terrorist planning," the FBI said in a statement.
If a solider, a Muslim unhappy about waging war on other Muslims, gets in touch with a man well-known for advocating terrorism, shouldn't that 'raise a red flag'?
BBC North America editor
There were between 10 and 20 communications, beginning in December 2008 and continuing in 2009.
Content was of a social nature as well as "religious guidance".
They did not include the sort of threatening or inciting language that would have triggered an investigation.
A senior government official said the "general tenor of the communications was benign".
The communications contained "no red flag", according to the official.
Senior investigators say Maj Hassan, who is conscious and has called for a lawyer, is to be charged in a military court, rather than the US District Court.
Investigators also said that being on the FBI's radar would not have been enough to prevent Maj Hassan from legally obtaining a weapon.
The FBI investigation has not identified a motive, and a number of possibilities remain under consideration.
Some reports said Maj Hasan, a 39-year-old US-born Muslim, was unhappy about being deployed to Afghanistan.
FBI Director Robert Mueller has ordered a review of how the agency dealt with information about Maj Hasan.
Senior US Senator Joe Lieberman has said he plans to open a congressional investigation into whether the shootings were a terrorist attack.
Mr Lieberman also said he hoped to determine whether the army missed signs that Maj Hasan may have harboured extreme views.
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In: Afghanistan, News
Tags: nidal Hasan, terror plot, FBI,
Location: Killeen, Texas, United States (load item map)
Marked as: approved
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