The Minneapolis police officer who used his knee to pin down George Floyd’s neck before his death was the most experienced of the four officers involved in the arrest, with a record that included medals for bravery and 17 complaints against him, including one for pulling a woman out of her car during a speeding stop. New details about Derek Chauvin and the other now-fired officers emerged Wednesday after prosecutors upgraded Chauvin’s charge to second-degree murder and charged the others with aiding and abetting in a case that has convulsed the nation with protests over race and police brutality. Heavily redacted personnel files show that Chauvin, a 19-year veteran of the force, was initially trained as a cook and served in the Army as a military police officer. Eleven-year veteran and native Hmong speaker Tou Thao began as a community service officer and was the subject of six complaints. The other two officers were relative newcomers to the department, including Thomas Lane, a former juvenile detention guard who did volunteer work with Somali refugees, and J. Alexander Kueng, who got his start in law enforcement by patrolling his college campus and a department store.