Invisible Target

An armoured truck carrying $100 million of cash is attacked by a criminal gang which calls themselves the "Ronin Gang" with explosives as it stops at a traffic junction in front of a jewellery shop. All the security guards were shot dead except one, while responding police officers suffered serious casaulties under the gunfire of the heavily armed gang members. The explosions also killed several civilians, including a lady in the shop. Six months later, the same gang reappears in Hong Kong, injuring a group of policemen, two critically, during a spot check. The dead woman in the jewellery shop was Crime Investigation Department detective Chan Chun's fiancee, while the injured policemen are led by Trainee Inspector Fong Yik-wei. Both officers vow revenge on the criminal gang.Wai King-ho is a rookie cop whose older brother, Wai King-tat, also a police officer, has been missing for an extended period. Deeply upset by police allegations that his brother has become a part of the criminal gang while infiltrating as an undercover, he was suspended from duty due to this connections with his now-wanted brother, and later confronted by both Chan and Fong who demand his brother's whereabouts. They form an alliance after Wai is rescued by Chan and Fong in a fight at a night club. Little Tiger is captured by Chan and is forced to lead the police to the gang's hideout. The gang appears however and kills Tiger after a chase, while injuring Chan and several of his colleagues.It turns out that the surviving security guard is Ho Wing-keung, the manager of the security company and who is in colludes with the gang to rob the armoured truck. He is tracked down by the gang to a psychiatric hospital, where he feigns a mental breakdown to escape detection by the gang. Chan, Fong and Wai arrive at the hospital, but fail to prevent his escape with the gang, who threaten his family to force him out. As the three pursue the gang, they run into an accident scene, where the gang shoots several police officers at the scene, hijacks a mini-bus with young children, and takes Wai as a hostage in the bus. Chan and Fong goes after the bus with Ho in another car.The gang members and the police officers engage in a lengthy confrontation, in which Wai discovers that his brother has his background exposed and killed by gang member Ronin Tien Yeng-yee. Ho is killed by Tien Yeng-seng to silence him, Wai is left handcuffed to the bus with the children and a timer bomb, while Chan and Fong are forced to collect the stolen cash from the robbery's mastermind at a funfair at Tamar while chained to another timer bomb. Wai manages to free himself and rescue all the children, and discovers that the bomb is fake, suggesting that Ronin has no intention to harm the children. Chan and Fong approaches a man with a luggage at the funfair as instructed, but the man turns out to be Chief Inspector Sam, who is leading an operation to nap the gang. The operation goes awry, the timer bomb is ignited safely in the nick of time, and Sam was pulled away by Chan and Fong to a secluded area, where they attempt to explain their situation to him. It is then revealed that Sam is also colluding with the mastermind, and is promptly killed, resulting in Chan and Fong becoming suspects for the murder.With the help of Fong's girlfriend Leung Hoi-lam from the Police Intelligence Department, Chan and Fong realise that the real mastermind is Senior Superintendent Cheung Man-yiu, who has orchestrated the armour truck robbery, double-crossed the gang resulting in the death of three of their members, and made off with the loot which he keeps hidden in a car in the Police Headquarters carpark. The remaining four members of the criminal gang arrive at the Headquarters in a police van masquerading as police officers to forcefully get the money back, just before the arrival of Chan, Fong and Wai. Senior Superintendent Mark Law Pui-keung begins to learn about the truth from data provided by Leung.An extended showdown at the Police Headquarters ensues, resulting in the deaths of three criminals, multiple policemen including a Special Duties Unit squad and Wai, who was beaten to death by Tien. An injured Tien was shot by Cheung, who nearly escapes arrest when he was later led to an ambulance by unsuspecting officers. He was subsequently shot dead by Chan and Fong when he tries to kill Law who was led to another ambulance. Both Chan and Fong return to their police duties, but as a detective and a beat cop respectively. They also help take care of Wai's grandmother.

Question: Who did the  remaining four members of the criminal gang arrive at the Headquarters masqueraded as ?
Answer: police officers