Later, he worked as a newspaper reporter for The Kansas City Star, New York World, the Chicago Tribune and the Denver Post.

His books Here's to Crime (1937), Ten-Thousand Public Enemies (1935) and Designs in Scarlet (1939) championed the cause of the young Federal Bureau of Investigation and made the case that corrupt local governments and police forces permitted lawlessness to flourish in many parts of the United States.

A 1936 newspaper article in the Chicago Daily Tribune identified Cooper as one of Hoover's few close personal friends, along with Clyde Tolson and a man named Frank Baughman.

He collaborated with Federal Bureau of Narcotics Director Harry Anslinger on the article "Marijuana, Assassin of Youth",[5] which originally appeared in The American Magazine in July 1937.

Mrs. Cooper,[7] of Los Angeles, could advance no reason for his suicide, but told police he had been morose over alleged snubs he had received in Washington when he sought to inform officials of German activities he said he discovered in Mexico.

Mrs. Cooper said her husband had made an exhaustive investigation in Mexico several months earlier and unearthed details of German conditions and propaganda there.