Businessman Datuk Ong Teik Kwong, who was shot dead by his bodyguard at the Tun Dr Lim Chong Eu Expressway, was detained under the Emergency Ordinance (EO) before.
Penang police chief Comm Datuk Abdul Ghafar Rajab said Ong was arrested several times by the police before and was one of the EO detainees in Simpang Renggam.
"Although he was detained by the police before, he was never charged with any crime.
"He was released after the EO was repealed in 2011," Comm Abdul Ghafar told a press conference after attending a Retired Senior Police Officers Association gathering at the Penang Officers Mess here.
He said that those detained under the EO were usually connected with cases involving violence, but they were not charged in court as police did not have enough evidence.
Ong, 32, and two other men were killed by Ong's bodyguard, who is believed to be mentally unstable on Dec 1.
Five other people were also injured in the incident, which took place at 7.30pm that day at the Tun Dr Lim Chong Eu Expressway.
Comm Abdul Ghafar said that the remand period of the bodyguard and another suspect, who was Ong's business partner, will end on Thursday (Dec 8).
He said police will be seeking an extension of the remand for both men for another week. He added the case is being investigated under Section 302 of the Penal Code for murder.
He added the gunman was the bodyguard to a "Tan Sri" in Kuala Lumpur before he started working with Ong.
Comm Abdul Ghafar said the police have so far recovered 11 bullet shells at the scene, and they will return to the crime scene today to complete investigations.
When asked if the police will carry out vetting on armed security guards and bodyguards, Comm Abdul Ghafar said the police would usually do a background check for any drug abuse or criminal activities, but not for their mental heatlh.