Moaning Myrtle said she made sure Olive Hornby was haunted by her memory of finding Myrtle's body until Olive's dying day. Olive had to seek help from the Ministry of Magic in an effort to be free of the spirit, so the Wizards there knew a witness (Myrtle's ghost) to the 1943 murder had become available. After the Ministry of Magic exorcised Myrtle, she was relegated to Hogwarts, generally haunting the same stall in the same lavatory she died in. The powers that be knew where she was
What does JKR say about why no one at the Ministry of Magic, or any of the Hogwarts Professor's, asked Moaning Myrtle about her death?