Americans think of Aaron Copland as their national composer. Leonard Bernstein called him "the best we've got", and Americans of widely varying backgrounds agree on the matter to this day. Works like Appalachian Spring, Rodeo, A Lincoln Portrait and Fanfare for the Common Man are American icons to vie with the Grand Canyon, Brooklyn Bridge and Mount Rushmore.

The connection with presidential monuments does not stop there. Presidents from Eisenhower to Carter sought to use Copland's music to accompany their inauguration ceremonies. Copland's aesthetic has become America's musical sense of itself. His harmonies and melodies have been endlessly imitated, especially in the movies, to embody emblematic American virtues of simplicity, goodness and love of wide-open spaces. Musically, he is the essential American.

But Copland was not just a great American composer. He was also a great American liberal composer. Politics and public affairs were at the centre of his life. He cannot be understood without at least some appreciation of the political milieu in which he grew up, came to maturity and lived for most of his life. Yet in recent years, Copland has been depoliticised. His essential liberalism has been increasingly downplayed in favour of a narrower version of his essential Americanism.

The publication this week of an FBI file on Copland, obtained by the Associated Press under a freedom-of-information request, is therefore a timely reminder of where Copland came from. His file begins with a document from 1950, identifying him as "thought to be self-employed as a composer of music" and ends with a 1955 memo giving his case"a pending inactive status". Even so, the FBI continued to compile information on Copland until as late as 1975.

The extent of Copland's political engagement is neither a secret nor a surprise. Copland never hid his essential political sympathies. But what these documents tell about the US treatment of Copland is as much the story of the harassment of 20th-century composers as anything that happened to Dmitri Shostakovich in the Soviet Union or to Kurt Weill or Ernst Krenek in Nazi Germany.

The climax of this process was Copland's 1953 testimony in front of Senator Joseph McCarthy's anticommunist hearings on "un-American activities". The hearings, which took place 50 years ago this month, consisted of a confrontational inquisition into every aspect of the then 52-year-old composer's political life and background. Though Copland probably never joined the Communist party, he was certainly steeped in the communist-sympathising culture of the era. But he was not going to give anything away.

Copland was more than equal to the challenge of the hearings, and he held his own with McCarthy in an often masterly piece of prevarication. "I spend my days writing symphonies, concertos, ballads and I am not a political thinker," he told the committee.

The FBI did not buy Copland's denials of communist involvement and they continued to monitor his movements. They seriously considered prosecuting him for perjury and fraud because he denied he was a communist. Some historians think they were probably right. In the 1930s, like so many people of liberal sympathies in the US and elsewhere, Copland mixed freely in communist and Marxist circles. In 1934 he wrote a song titled Into the Streets May First.

The same year he made a speech supporting the Communist candidate for governor of Minnesota. Two years later he supported the Communist candidate for president, Earl Browder, against Franklin Roosevelt. For many years Copland was strongly pro-Soviet. "He was involved with the Communist party up to his ears," the distinguished New York music critic Terry Teachout told AP this week.

Copland's communist sympathies were very much of their time. His identification with the people was at the root of his own aesthetic and of his view of the role of the artist. "Musicians make music out of feelings aroused out of public events," he boldly told his Senate inquisitors in 1953.

This commitment - communist, populist or liberal- is at the root of much of Copland's art. In truth it is probably more important to an understanding of Copland as an artist than the now much more fashionable interest in his homosexuality. In his authoritative 1999 biography of Copland, Howard Pollack quotes the composer as saying: "The artist should feel himself affirmed and buoyed up by his community. In other words, art and the life of art must mean something, in the deepest sense, to the everyday citizen. When that happens, America will have achieved a maturity to which every sincere artist will have contributed."

To the end (he died in 1990), Copland remained a man of the people and a man of the left. But he always knew where to draw the line. As Pollack also records, when Copland received an official telegram from President Nixon congratulating him on his 70th birthday, two friends rolled marijuana in it and suggested making a joint out of the greeting. But Copland had other ideas. "It's from the president of the United States. That goes in my scrapbook."

· More details of the Copland FBI files are at datacenter.ap.org/wdc/copland/start.html