An economics professor at Thammasat University said today that the four unidentified assailants, captured on CCTV attacking anti-junta activist Sirawith Seritiwat, or Ja New, on Friday, used a blunt weapon which he suspects was a type of telescopic metal baton called a “blackjack”.

Associate Professor Suvinai Pornawalai said on Facebook today that a person who could use a blackjack effectively must have been trained in its use and might be a policeman, military personnel, security guard or a hired hooligan.

According to Wikipedia:

A blackjack is a short, easily concealed club weapon consisting of a dense (often lead) weight attached to the end of a short shaft, used as a bludgeon.  The weapon works by transferring kinetic energy to the dense core, via the handle, during the swing. When directed at the head, it works by concussing the brain without cutting the scalp. This is meant to stun or knock out the subject, although head strikes have a high risk of causing a permanent, disabling brain injury or being fatal.

Most modern police departments have phased out the blackjack from their standard issue equipment and most banned their use entirely as it is considered potentially deadly.