Police in Seoul are searching for a man who sent a mail bomb to the head offices of CJ Entertainment as part of an extortion scheme.

The small explosive, concealed inside a book addressed to CEO Lee Kang-bok, caused minor burns to Lee's hands and forehead.

A note pasted on the inner cover of the book revealed the suspect to be the same man who called in a bomb threat in early December at four multiplexes owned by CGV, a subsidiary of CJ Entertainment. The call resulted in the evacuation of the four cinemas, after which small parcels were found with detonators but no explosive material.

The suspect has threatened an escalation of attacks if CJ does not pay him a total of $16,700 (WON 20m). Precautions have been taken at CGV theatres, with bomb experts carrying out thorough searches between shows.

The events have caused a stir in a country where such crimes are almost unheard of. However, no drop in attendance has been reported in the four cinemas originally targeted by the suspect.

Police have offered a reward for the suspect and have installed CCTV cameras in select post offices to assist in their search. The relatively small amount of money demanded has led investigators to speculate that the suspect is an individual motivated by either a grudge or a debt.