Farouk Arnaz, Anastasia Winanti Riesardhy & Robertus Wardi – More indications have emerged that point to a rogue military unit as the heavily armed group who infiltrated a police detention center in Yogyakarta last week and shot dead four detainees.

Brig. Gen. Boy Rafli Amar, a spokesman for the National Police, characterized the actions of the group of 17 masked gunmen as highly disciplined and professional.

"You could say that it was [well-planned] because it was carried out carefully, systematically and quickly," he said in Jakarta on Wednesday.

He added that police investigators were also looking into witness claims that the perpetrators had used a special code in speaking to one another during the raid on the Cebongan detention center in Yogyakarta's Sleman district last Saturday.

"We can't reveal what the [code words] were because we're still investigating," Boy said.

During the raid, the gunmen stormed the jail, disarmed the police officers on duty, shot and killed four detainees who had only recently been transferred there, then took the CCTV recordings before making a clean exit.

The clinical nature of the attack, combined with the fact that the four victims were all suspected of the murder of an Army Special Forces (Kopassus) officer, has sparked speculation that the incident was a revenge attack by the military.

Police have confirmed that spent shell casings found at the scene were from 7.62-millimeter rounds, the same kind in the AK-47 assault rifles used by Kopassus, which witnesses have said the gunmen were wielding.

Army and police leaders have denied the allegation that the assassins were from the military, but a former armed forces commander has hinted at the possibility.

Wiranto, the former military chief of staff and now a prospective presidential candidate, called on Wednesday for "leaders to look within their own institutions" for the perpetrators, in a perceived swipe at the current batch of military leaders.

"To resolve the Sleman case, the leaders must be honest and hand over the guilty to face justice," he said. "We need leaders to be honest and to look within their institutions and admit to the public whether their own subordinates were involved."

He added that the descriptions of the weapons alone should compel all security forces to carry out an internal inquiry. The Defense Ministry, however, has denied that Kopassus carried out the hit, calling the attack the unprofessional work of an armed criminal group.