McQ

It is just before dawn in Seattle. A man dons dark glasses and gloves and loads a 9mm silenced automatic handgun. He drives into town, where he shoots a policeman (Officer Philip Forsell; in the film the character is identified only as Hyatt) on his beat, then drives to a police impound yard and shoots the officer on duty (dialogue identifies him as Wally Johnson). At a luncheonette, as he washes his hands, he momentarily flashes a police badge belonging to Detective Sgt. Stan Boyle (William Bryant). When a car pulls up, Boyle goes outside and gives the driver a satchel containing the 9mm and proceeds to his own car â but is shot in the back by the unseen driver.
Seattle Police Department, and the head of the homicide investigation, Captain Edward Kosterman (Eddie Albert), believe the shootings are the work of street militants; Kosterman orders an immediate dragnet.
Elsewhere, Detective Lieutenant Lon "McQ" McHugh (Wayne) escapes an attempt on his life by a professional hit man named Samuels. McQ had been woken minutes before by a phone call to him on his boat, telling him of the shootings of his longtime partner and the two other police officers.
Because he and Boyle had been investigating drug trafficking in the city, McQ is convinced from the start that the target of their investigation, local shipping magnate and known drug dealer Manny Santiago (Al Lettieri), is responsible for the shootings.
Despite a warning from Captain Kosterman to leave the investigation to the department, McQ, after talking with Boyle's wife Lois (Diana Muldaur), gets behind the wheel of his personal Pontiac Firebird car and begins tailing Santiago. After seeing a TV news report that Boyle has died of his injuries, he rages after Santiago and beats him viciously in a men's room.
When confined to desk duty by Kosterman, McQ angrily resigns, despite pleading from fellow detective Franklyn Toms (Clu Gulager). Continuing to investigate the case through a partnership with local private eye "Pinky" Farrell (David Huddleston), McQ learns that Santiago has assembled a heist team to steal the confiscated heroin and cocaine from the police department's evidence vault. The drugs are normally held by the department until turned over to the State Attorney General's Office for disposal. Santiago's men steal the drugs just as the narcotics are about to be burned in an incinerator. McQ pursues Santiago's men, but they escape. After getting a much harsher warning from the increasingly exasperated Kosterman, McQ acquires for himself a MAC-10 submachine gun.
McQ breaks into Santiago's office but is cornered by Santiago and his men. Santiago reveals that the drugs his men stole had turned out to be only powdered sugar. The real drugs, from hundreds of major and minor cases and investigations, had been carefully, over a period of years, replaced with the sugar. Obviously this could not have been done without extensive corruption throughout the department. McQ also realizes that Santiago was not responsible for Stan Boyle's death. Knowing McQ is not a threat, Santiago lets him go â though he beats him brutally as payback for the earlier assault.
McQ's investigation leads to the shooting of one of his sources, bartender Myra (Colleen Dewhurst), and another attempt on McQ's life, in which his Firebird is crushed between two huge trucks. McQ escapes, but when he examines the wreckage he finally discovers who is behind the killings of Boyle and two other officers, and also who is behind the theft of drugs from the police, leading to a climactic chase and shootout at a beach with Santiago and his men.

Question: Who does Kosterman believe committed the shootings?
Answer: Street militants.