Police officers are the second-biggest killers in the state of Utah, outpacing gang members and drug-dealers, a new report found.

Law enforcement in the state killed have 45 people since October 2010, second only to domestic violence, according to the Salt Lake Tribune.

The death count - 15 per cent of all homicides in that time - puts them ahead of gang and drug-related violence and child abuse. In 2014 alone, 13 people have been killed by officers.

Killings: Police in Utah are responsible for 45 deaths since 2010, a new report found. Pictured are Utah Highway patrol training with grenade launchers near Camp Williams

Of the 45 cases where an officer killer somebody, only one made it to court.

Officers in West Valley City shot dead Danielle Willard after catching her buying drugs. The district attorney pressed charged, but they were thrown out by a judge.

The other 44 deaths were not deemed worthy of criminal charges by local prosecutors.

The Tribune's study - based on news reports, public documents and official crime statistics - has sparked calls for reform in officer training and a potential changes to rules about when police use deadly force.

Justice professor Robert Wadman, of Weber State University, told the Tribune: 'Sometimes the line between "is it legal?" and "is it necessary?" becomes difficult to distinguish.

Necessary? Questions have been raised over how officers are using their right to deadly force. Pictured above is a SWAT team responding to a gunman in 1999

'In the judgment of the officer: "Is my life in jeopardy? Yes." At that point in time, they’re legally grounded in using deadly force. But the question is, is it necessary? That’s something that needs to be firmly addressed, for example, in training.'

However, a spokesman for Utah's police union said the deaths are a result of deadly situations officers are put in, and that it is criminals' responsibility not to make officers shoot them.

Ian Adams said: 'Police are trained and expected to react to deadly threats... The onus is on the person being arrested to stop trying to assault and kill police officers and the innocent public.