Tour bus driver Alberto Lubang, who escaped from the vehicle as the hostage-taker started shooting at passengers, is being investigated as a possible accomplice of the gunman.
Manila mayor Alfredo Lim said police are investigating a possible "tie-up" between hostage-taker Rolando Mendoza and the driver.
Lim said 38-year-old Lubang acted "weird."
"He was not afraid, he was very friendly toward Mendoza, and he was just laughing," Lim said yesterday.
He said a normal hostage would not act so casually, and he described Lubang's claim that he cut through his handcuffs with a nail file as absurd.
Lubang said he could not believe he escaped despite being handcuffed to the steering wheel.
He said he used the file on a nail cutter, which he carries to work, to free himself.
"I sawed at my cuffs. And when they broke I escaped."
On Monday, Mendoza's brother, Gregorio, was arrested on obstruction of justice allegations. The traffic cop was intercepted after he attempted to approach the bus, armed with a gun.
His presence was blamed for agitating the hostage-taker, for minutes after he saw his brother being taken by police, Rolando Mendoza started shooting.
Philippine National Police spokesman Agrimero Cruz, promised an internal investigation of the police handling of the crisis. "We will look at whether what we did was right," Cruz said.
Meanwhile, a Philippine lawmaker yesterday proposed a news blackout during hostage crisis situations, saying live coverage may have jeopardized the police operation.
"Care should be taken to avoid disclosure of vital information to the perpetrators until the crisis situation is resolved," Luis Quisumbing said in putting a bill to the House of Representatives.