Classification has impeded investigations, particularly civil investigations, but generally the classification already exists for legitimate (or at least well-intentioned) reasons. Someone asks for info, which happens to be classified, they are told no (correctly) and now it's a "cover up".

Certain components within the government certainly may have enjoyed collateral benefit from the routine classification of info which turned out to be controversial down the road, but it would be very difficult to create a classification solely to cover something up. For one thing it would be illegal, and for another only the people at fault stand to benefit...anybody else with knowledge who was NOT at fault would risk their career, pension, and possibly freedom by collaborating to cover something up. People with nothing to gain but lots to lose won't play along, certainly not all of them.