Members of the Green Street and Bloods gangs allegedly pledged to retaliate against the police officer who shot and killed John Earl Parks of Brockton on Nov. 5, US Attorney Michael J. Sullivan said in a written statement.After Brockton officers stopped Parks in an alleged drug deal near Warren Avenue and Green Street, Parks fled on foot and fired a gun at officers, Police Chief William K. Conlon said. Officers returned fire, killing the 25-year-old.After learning of the alleged plot to target the officer, Brockton police arrested 12 suspected gang members on gun and drug-related charges yesterday.Eight Brockton residents - John Goncalves, 22; Placido Pereira, 22; Dennis Afonso, 25; Emmanuel Teixeira, 18; Rodney Gallaway, 29; Embassi Dosanjos, 24; Johnny Richmond, 21; and Daquawn Jones, 18 - were charged with federal offenses ranging from conspiracy to possess with intent to distribute crack cocaine to distribution of crack cocaine within a school zone, Sullivan said. If convicted, the men face up to 20 years' imprisonment, Sullivan said.Four other Brockton men - William Hart, 36, Angel Otero, 19, David Stuart, 18, and Edson Miranda, 29 - were charged on the state level with distributing crack cocaine or possession with intent to distribute crack cocaine, Sullivan said. The men could face up to 10 years in prison if convicted, the US Attorney said.Conlon said an investigation monitoring violent and drug-related gang activity in the city began earlier this year and would have continued were it not for "imminent danger" against police."It's a relief for our residents," Conlon said. Gangs "have been a plague on the neighborhoods, and the good people of city deserve better."Plymouth District Attorney Timothy J. Cruz said the arrests send a strong message that the community stands with its police officers."We will not tolerate a threat to their safety or to the law-abiding people who they serve," Cruz said.The State Police Gang Unit and Federal Bureau of Investigation, in addition to Brockton police, investigated the cases.FBI Special Agent Warren Bamford emphasized the severity of the charges against the gang members. "The FBI, along with its law enforcement partners, will not rest until we root out the gangs that terrorize our communities," Bamford said.