Oswald was a impoverished failed half assed Marxist who was seen as unstable and unreliable a good reason he was allowed to leave the Soviet Union to be of more use to the cause of Marxism as a troublemaking Marxist in the USA like the "peaceful protestors" described above
The same circumstances were in place where Oswald was under surveillance but strangely they slipped up on following what he was doing , of course there were no federal laws to prohibit mail away firearms then so there could be no official acts taken to warn gun dealers in the zone to contact federal officals if Oswald did buy a firearm , also due to the fact there was no federal 4473 in use Oswald did not have to answer question 21 J about renouncing his US citizenship a question that was put on the 4473 in 1968 when the gun control act of 1968 became law because of the Kennedy assassination and the killings of Dr King and Bobby Kennedy in 1968
Oswald went to Mexico to get the Cuban visa as he thought he would get back to the USSR from there, he was a failure as a father and husband as a provider to his family , he was rebuffed by the Soviets in Mexico and when he came back to Dallas and found that Kennedy was coming as it was advertised on radio and TV and in newspapers with the president's motorcade route from Love field described in maps in newspapers ,he saw his job position and location provided a perfect opportunity in the TSBD to set up a snipers nest , he acted alone in shooting President Kennedy escaping and killing officer Tippet thinking he could make it to Mexico and Soviet contact's there would smuggle him back to the USSR as his Marxist reward , he was apprehended and then gunned down by Jack Ruby who had many police friends as a strip club operator in Dallas who got his access into the garage as a "cops good buddy" .
Jack Ruby thought he was doing his country a great deed in a moment of passion seeing his position to Oswald in the parking garage when he suddenly lunged forward calling Oswald a "son of a bitch" and shooting Oswald in his midsection killing him, when in fact he only made Oswald's deed the subject of intense conspiracy theories forever into history
The fact so many articles were covered up and sealed added to this , but as I said above this may have been the intended response. Another great factor was police work was not as professional then as it is today and that was present in federal law enforcement as well as state then . There were far to many sloppy handled events such as Kennedys body being taken by the Secret Service despite a show down by the Texas official DR Earl Rose the Dallas medical examiner who had the power of law behind him to do Kennedys autopsy which was done in Bethesda MD by the Navys doctors who did not have the pathologist forensic crime evidence training Rose did so Kennedys neck wound was rendered unable to determine exactly how it occurred . In many ways it all was a real case of a "Cluster F " all the way https://www.youtube.com/watch?v=R76MK1M9MOs
DR Rose youtube after L.H Oswalds autopsy