ALBANY, N.Y. -- As Army Pvt. Pete Seeger eagerly waited for a chance to fight for his country during World War II, military investigators quietly built a case the young folk singer was "potentially subversive."

In a security investigation triggered by a wartime letter he wrote denouncing a proposal to deport all Japanese Americans, the Army intercepted Seeger's mail to his fiancee, scoured his school records, talked to his father, interviewed an ex-landlord and questioned his pal Woody Guthrie, according to FBI files obtained by The Associated Press.

Investigators concluded Seeger's association with known communists and his Japanese-American fiancee pointed to a risk of divided loyalty.

Seeger's "Communistic sympathies, his unsatisfactory relations with landlords and his numerous Communist and otherwise undesirable friends, make him unfit for a position of trust or responsibility," according to a military intelligence report.

The investigation, forwarded to FBI director J. Edgar Hoover, is detailed in more than 1,700 pages from Seeger's FBI file, released by the National Archives under the Freedom of Information Act.

The musician and left-wing activist known for such songs as "If I Had a Hammer," "Where Have All the Flowers Gone" and "Turn, Turn, Turn" died in January 2014 at age 94.

The government's interest in Seeger is no surprise. He was a member of the Communist Party for a period of time, as he later acknowledged, saying he quit around 1950, and he performed with Guthrie and others in the proudly leftist Almanac Singers before he was drafted.

But the newly released files show the lengths to which the government went to keep tabs on the singer's travels, performances and rally appearances at least into the 1970s. The archives plan to release additional Seeger files in the future.

The degree to which the security investigation affected his military career is not spelled out in the files. Seeger trained as an airplane mechanic at Keesler Field in Mississippi, and according to his file, expressed frustration he was being kept stateside instead of being sent to where the fighting was.