Ten days before KAL 007, an arctic storm wrecked all of the long range radars in the Soviet Far East. The Soviet Air Defense Forces were supposed to be able to repair and restore to full working order those radars within 48 hours. They couldn't because all of the critical spare parts had been stolen and sold on the black market.

Two days later, the Politburo asked the commander of the Soviet ADF Far East forces whether the radars had been made operational yet. He lied, and said yes.
Eight days after that, KAL 007, thanks to (apparently) a badly programmed navigational computer, strayed over Soviet airspace on the Kamchatka Peninsula. While local tactical SAM radars could pick up the 747's general vicinity, they couldn't target an aircraft moving at that speed and altitude that wasn't heading for any specific Soviet targets.
Soviet fighters looked for the 747, but since they had restricted fuel capacity (to prevent them from flying to Japan) and very restricted radio frequency sets (to prevent them from talking to any non-Soviet source), they had a very hard time trying to find them without the long range radars that were supposed to direct them in.
Only as KAL 007 was leaving Soviet airspace did the Soviets determine that the aircraft had no Soviet IFF transponder. Which meant, to the Soviets, "if you ain't us, you da' enemy". The rest was infamy.
That the Soviets, after hashing out the truth for themselves, would have doubled down on their own lies is no surprise. Most of the Soviet Politburo members would be dropping like flies within the next two years anyway, including the Soviet Minister of Defense Ustinov who gave that press conference worthy of the Ministry of Love.
Soviet claims of an American recon aircraft being the "real plane" was patently false, as the Soviets never actually knew the USAF aircraft was in the area (it was outside of their remaining working radars), and the plane in question had been down on the ground for over an hour by the time KAL 007 was shot down.
Michael Moriarty starred in a film playing an actual USAF Intelligence major who went to his general to tell him that in his professional opinion the Soviets had good reason to suspect that KAL 007 was the same USAF recon aircraft, and the general went to a Presidential intelligence briefing with that information. The general, and later the major, got crucified. The film ended leaving the viewer thinking that it was all "just a misunderstanding".
After the Wall came down, the records revealed that the film-makers were WRONG and the Reagan Administration was RIGHT.