WASHINGTON: The FBI is investigating more than 100 suspected Muslim extremists in the US military, following the 2009 “lone wolf” attack by an alleged al Qaeda sympathiser that killed 13 people at a Texas army base, a news report said Monday.

About a dozen of the cases are considered “serious” threats involving suspects believed to be actively planning attacks or in contact with dangerous extremists, National Public Radio reported.

The potential internal threats come from active duty military as well as reserves and civilians with access to military installations, the radio broadcaster reported.

The Pentagon declined to comment on the investigations.

The FBI discussed the investigation at a joint House and Senate committee closed-door hearing last December, according to NPR.

“I know one can say that as a percentage of the millions of people in active military service or working with contractors, the numbers you talk about are a small percentage of the total,” Joseph Lieberman, the senator who presided over the hearing, told NPR.

“But the reality is it only took one man, Nidal Hasan, to kill 13 people at Fort Hood and injure a lot more.” In November 2009, US Army Major Nidal Hasan opened fire on his fellow soldiers at a base in Fort Hood, Texas, killing 13.

An investigation revealed that Hasan was in contact with Anwar al-Awlaqi, a radical Yemeni American imam tied to al Qaeda.

Al-Awlaqi was killed in an American drone attack in Yemen in September 2011. Hasan's martial court trial has been set for August 20.