Police are hunting down the mastermind who commissioned a teen to spray paint 44-meter-long graffiti on the walls of a historic palace in central Seoul earlier this month, officials said Tuesday.

A 17-year-old, only identified by his surname Im, was arrested last week after spray painting the phrase "free movie" in Korean on both sides of the western gate to Gyeongbok Palace and palace walls near the National Palace Museum of Korea on Dec. 16.

Investigators have since found that Im was paid 100,000 won to commit the crime.

In an effort to track down the mastermind, police have conducted digital forensics on Im's mobile phone, looked into the bank account used for the crime, and are scrutinizing the dialogue the two had on the messenger Telegram, according to national police chief Yoon Hee-keun.

But officials said they are having difficulty tracking down the mastermind due to strong security features of the messaging app and its server being based overseas.

The police chief vowed to deploy patrols near the cultural heritage sites during vulnerable time periods and install surveillance cameras in blind spots subject to crimes in an effort to prevent similar copycat crimes from happening in the future.

The Seoul Metropolitan Police said it will beef up patrols near the five palaces in central Seoul, including Gyeongbok Palace, by arranging squad cars during the daytime, and additionally deploying detectives and the riot police forces at night until the end of this month. (Yonhap)