Which attorney general created a red scare in 1920 by arresting 5000 suspected communists and anarchists who were held without trial and denied their basic civil rights?

That would be general Alexander Mitchell Palmer.
Alexander Mitchell Palmer. was elected as an Attorney general under President Woodrow Wilson in 1919.
He was known as the leading figure in Palmer raids, an operation by Departement of justice to capture suspected Communists between the period of 1919 ad 1920.