While the White House and Attorney-General Robert Kennedy, the then President's brother, were sympathetic to King's cause they too had felt some apprehension about the potential for trouble.
Presidential worries might be assuaged by the facts of peaceful protest but facts were not enough to sooth the demons and suspicions in the mind of J Edgar Hoover, the director of the Federal Bureau of Investigation.
Hoover believed that blacks organising and demanding, equality - the civil rights guaranteed them under the US Constitution - were thereby part of a communist conspiracy.
He instructed agents under his command to spy on King and several march organisers.
Hoover eventually sent King's wife illegally obtained evidence of her husband's infidelity, but only after sending King anonymous threats to reveal his extramarital activities, suggesting that he commit suicide.
That these actions were unlawful did not seem to occur to Hoover.
After all, he had gained his start as a 23-year-old in WWI, rounding up suspected enemy aliens and jailing them without trial under authority derived from the 1917 Espionage Act.
For more than five decades, Hoover conducted his investigations into those he deemed disloyal.
He did not confine himself to foreign-influenced groups.
Hoover amassed potentially embarrassing information by which he held several US presidents in blackmailed thrall, thereby maintaining his powers and job.
It should be of little surprise that his staunch anti-communism led him after WWII to spy on civil rights groups and, after the start of the Vietnam War, to apply his extra-legal methods to spy on and disrupt anti-war dissidents.
After Hoover's death in 1972, former FBI agents Wesley Swearingen and Jack C Ryan apologised for their roles in spying on, and attempting to disrupt, groups engaged in anti-war and other legal dissident activities, under orders from Hoover.
Of those few agents convicted for these illegal activities, none served time in jail as they received presidential pardons from Ronald Reagan.
This week, many will honour King and his speech and the march which helped bring progress to the status of blacks. President Barack Obama acknowledged his own debt to the civil rights movement in 2007 while still a senator seeking the votes of African-Americans.
His speech this week about the march and Dr King will come from a President who owes his current role to that movement and to the march that exemplified it.
Obama is also the man who has presided over the largest expansion of the surveillance apparatus in US history. Obama's Attorney-General, Eric Holder, who happens also to be an African-American, has prosecuted more whistle-blowers and leakers than under all previous presidential administrations.
The law under which those prosecutions continue to be pursued is the same one that gave J Edgar Hoover his start; the 1917 Espionage Act.
Martin Luther King is often quoted saying that the arc of history is long but it bends towards justice.
Apparently, it also bends towards irony.
