SEOUL – It began with a simple phrase -- "brace yourself" -- sent by a university professor to his mistress via a South Korean messaging service app used by millions every day.

But now the message has become key evidence in a murder investigation, with the professor arrested for killing his wife and his mistress sought as an accomplice, police said.

The professor, whose full name was not given, went to the head office of the "Kakao Talk" messenger service provider after the murder and asked them to delete the message. But the message was saved for a month and police were able to retrieve it as part of an investigation of his phone records.

Eventually the professor confessed to strangling his wife, using his mistress's car to transport the body, and dumping it into the sea. He and his wife were in the process of getting a divorce.

"The message helped us discover that the woman was his accomplice," a spokesman at a police station in the southern city of Busan said. A warrant has been issued for her arrest.

"Kakao Talk" is a relatively new free smartphone app with over 14 million users that allows them to send messages, chat, take part in group chats, and send photos.