Both the CIA and the FBI witheld important information from the Warren Commission. The CIA didn't tell them about "Operation Mongoose," the plan to use the Mafia to help get rid of Castro, nor did they tell them about their surveillance of the Cuban and Soviet embassies in Mexico City--where they had double agents and had observed Oswald's visits in September 1963.
In the PBS documentary
Who was Lee Harvey Oswald?, one of the Warren Commission's attorney's states that the CIA let him listen to parts of the surveillance tape of Oswald at the Russian/Cuban embassy in Mexico. So that means that they did inform the Warren Commission of their operation in Mexico City and they also confirmed Oswald's visit even though they showed pictures of another man who they claimed was Oswald...
