(shouting) >> reporter: those who kne matthew riehl knew he was mentally ill but never imagined him o be a killer. last new year's eve in a denver suburb, after he called 9-1-1 aiming that his roommate verbally assaulted him, riehl spun out of corol, barricaded himself in his room with guns, and was judged by deputies to be a threat to himselfthers. that is colorado's standard for forcing voluntary mental health treatment. but when they tried to take him into custody... >> open the door! sheriffs office! (gunfire) hundreds of rounds were fired. one deputy, zach parrish, died. fheour were wounded along with two civilians in the apartment complex where he was holed up. ( gunfire ) in a subsequent shootout with a swat team, matthew riehl also died, ending his long descent into mental illness. >> he did very good in school. he graduated from c.u. denver magna cum laude. >> reporteer graduating, he moved to wyoming, where he loved to hunt and fish, and he enrolled in law school. his future was bright. he edhad friends, he join the national guard as a medic, eventually deploying to iraq. te

do you believe that if you had a red flag law prior to this incint, that both your deputy and matthew riehl might be alive today? >> i do, becaus had it, we would have taken him reto custody when he was ening the lone tree police department. >> reporter: opponents of such a law killed a measurethn colorado is year with the help of the gun lobby, charging that the due process rights of the mentally ill whose guns could temporarily removed would be violated. and that the rights of all gun owners could be in jeopardy as they see "red flag" laws as a move toward broader gun control. but for the republican sheriff whose depues were killed and wounded, it is about public safety. >> and if we had had it, we could have intervened en, and we could have done a lot of different things that we were not allowed to do, based upon the law. >> reporter: in colorado, proponents vow to retroduce a new bill next year, warning that, in the interim, there is a continuing risk of more fatal shootings involving mentally ill persons whbe a threat to themselves or others. for the newshour, i'm john ferrugia in denv