Human rights activists blamed the National Civil Police of El Salvador for well over one hundred "extrajudicial executions" between 2014 and 2018.
In the fight against gang violence at least 116 people were "brutally murdered" during this period, said the office of the Ombudsman for Human Rights in the capital San Salvador.
Attorney for the office, Raquel Caballero de Guevara, said a 141-page report describes patterns suggestive of "extrajudicial executions and excessive use of force" by the police. Some of the 48 cases investigated have found evidence of torture.
This must be investigated so that those responsible could be brought to justice, she said. There was "strong evidence" that the suspected gang members were not - as alleged by the police - killed in armed clashes, but by killing. Of the cases examined, only 19 have been tried in court so far, only in two cases have there been convictions.
The Central American country has one of the highest murder rates in the world. Last year there were 51 murders per 100,000 inhabitants - but that was only half as many as three years earlier.
President Nayib Bukele, who took office in June, called the fight against gangs and the reduction of violence in the country the priorities of his presidency. The Office of the Ombudsman for Human Rights was set up under the 1992 Peace Accords, which ended the civil war, which had been raging since 1979.