Walker v. City of New York

Michael Walker (plaintiff) sued the City of New York and police officers Gregory Gordon and Michael Smith (defendants), alleging that the officers used excessive force by shooting Walker during a 2013 encounter. The officers claimed they shot Walker to defend themselves and the public. At trial, the officers sought to introduce DNA evidence from a firearm found near the shooting, claiming that Walker had been wielding the weapon. The DNA sample from the gun was extremely small, consisting of only 22.35 picograms of DNA, and was mixed, meaning that it contained DNA from more than one person. The sample was processed by New York’s Office of the Chief Medical Examiner (OCME) using its Low Copy Number (LCN) testing and Forensic Statistical Tool (FST). LCN was a process developed by OCME to analyze low-level DNA samples. Although the New York State Commission on Forensic Science (commission) approved OCME’s use of LCN in 2005, LCN’s reliability remained controversial, particularly in relation to small samples. No other American laboratories adopted LCN. FST was software developed by OCME to calculate likelihood ratios for DNA samples containing two- and three-person mixtures. The likelihood ratio stated how likely it was that a suspect’s DNA was part of a mixture. The commission approved OCME’s use of FST in 2010. After processing the sample from the firearm using LCN and FST, OCME concluded that the sample was a two-person mixture and that it was approximately 14,200 times more probable that the sample originated from Walker and one unknown person than from two unknown, unrelated persons. Walker moved to exclude expert testimony regarding OCME’s findings, asserting five arguments, one of which was that the officers had failed to show that OCME’s methods were sufficiently reliable for determining the number of contributors in a mixed DNA sample of less than 25 picograms. In support of the methods’ reliability, the officers pointed to the general validation studies done by OCME when the methods were developed, the commission’s approval for the use of LCN and FST, and one third-party study. However, the third-party study noted that the study’s analyzed criteria were significantly less reliable when applied to small samples. The district court considered the parties’ arguments.