Police in Oklahoma kill more people per capita than police in any other state, according to data compiled about police killings since the start of 2015. Arizona police kill citizens at the second-highest rate.

No federal body keeps a tally on the number of people killed by police in the US, but both The Guardian and the Washington Post began recording police killings this year.

Since the start of the year, police have killed 22 people in Oklahoma, or about 4.4 people per million, according to numbers from The Guardian, which include deaths following car chases and incidents in which police were fired at first.

Arizona has the second-highest rate of police killings in the US, with about 3.6 people killed per million. Most other states have a police-killing rate between one or two killed per million residents, Al-Jazeera America reported.

Police in the US are on pace to kill nearly 1,000 people in 2015. The Guardian has recorded 465 police killings and the Post has recorded 385. The Guardian includes all death caused by police, while the Post looks at only shooting deaths.

The Oklahoma branch of the American Civil Liberties Union points to a lack of training on how to deal with mentally ill people as to why that state’s police force is leading the US in killing of residents.

“One of the biggest issues in Oklahoma is that we see a lot of departments with very little training and experience in dealing with folks with mental health issues or who are under the influence of substances,” Brady Henderson, the legal director for ACLU Oklahoma, told Al-Jazeera. “Dealing with someone like that can be dangerous, but if officers are properly trained and the community has the right resources, there’s no reason someone should die in that situation.”