The Texas High School gunman taunting his victims. A new toxic danger in hawaii as the volcano pours into the pacific. And diagnosing adhd in kids using brain scans. Tonight the signs doctors say parents should look out for to know when a child should be tested. Announcer this is nbc nightly news with lester holt. Good evening and welcome. We begin with a tense situation thats been unfolding all afternoon long in Baltimore County, maryland. A female Police Officer killed and a neighborhood, the scene of an intense manhunt. Schools were put on virtual lockdown and workers and residents warned to shelter in place. The killing of the officer during an apparent traffic stop brings to 56 the number of Law Enforcement members killed on duty so far this year around the country. Pete williams has late details. Reporter in the air and on the ground, state and local police launched a manhunt in a suburb of baltimore, maryland after a Baltimore County Police Officer was killed. Can you let us kno