In the film Memento Leonard's main tattoo reads
JOHN G. RAPED AND MURDERED MY WIFE
That means it was the second attacker who raped his wife. If this was true the postmortem report would've shown that the DNA of the samples found on her body did not match with the first attacker. Even if she survived the attack forensic investigation when she was undergoing treatment would've revealed this.
This should have made the cops search for the second attacker. Why didn't this happen?
If we assume this second guy was smart enough to remove all evidences off Mrs. Leonard's body then how did Leonard know that his wife was raped? Because when Leonard wakes up in the middle of the night and shoots the first attacker he only sees a masked man strangling his wife.