A Washington state law aimed at improving police accountability is under scrutiny after three Tacoma officers were acquitted in the 2020 death of Manuel Ellis, a Black man who was shocked, beaten and restrained facedown on a sidewalk as he pleaded for breath.

The measure approved by voters in 2018 was designed to make it easier to prosecute police accused of wrongfully using deadly force. Initiative 940, referred to as I-940, removed a requirement that prosecutors prove an officer acted with actual malice in order to bring a case – a requirement no other state had – and established that an independent investigation should be conducted after use of force results in death or great bodily harm, among other things.

The nearly three-month trial of the three police officers – Matthew Collins, 40; Christopher Burbank, 38; and Timothy Rankine, 34 – was the first to be held under the five-year-old law. The trial over Ellis’s death in Tacoma, about 30 miles (50km) south of Seattle, ended Thursday with their acquittal on various murder and manslaughter charges.

The acquittal came a day before a jury in Colorado convicted two paramedics in the death of Elijah McClain, another Black man whose case drew national scrutiny.

Matthew Ericksen, a lawyer for the Ellis family, said Washington’s 2018 police accountability law failed in certain regards in a trial that amounted to a test case for the measure, resulting in a verdict that devastated the family.

“One of the big reforms that I-940 was meant to bring was completely independent investigations of in-custody deaths like Mr Ellis,” Ericksen said. “And that just didn’t happen. The law was violated, and in many ways, there really haven’t been any consequences for that.”

The Pierce county sheriff’s office botched the initial probe into the death by failing to disclose for three months that one of its deputies had been involved in restraining Ellis, despite the state law requiring independent investigations. The Washington state patrol took over, and the state attorney general’s office conducted its review based on evidence gathered by the patrol as well as its own additional investigation before charging the officers.

How effective I-940 can be will come down to how it is enforced, according to Ericksen. While the Ellis case highlighted gaps in the measure, he said it remained one of the “necessary building blocks to hopefully get to some police accountability”.

“We’re better off having I-940 than not,” he said. “I sincerely hope this one verdict does not deter future investigations and prosecutions, and I know the Ellis family feels the same way.”

Other police-reform advocates also were disappointed by the verdict but said the fact the case went to trial at all – due to the law dropping the requirement that prosecutors prove officers acted with malice – already marked a significant change. Another Washington state officer, Jeff Nelson in Auburn, south of Seattle, is awaiting trial on a murder charge brought after I-940.

“We made it possible for them to know that you can be charged if you do something wrong,” said Tonya Isabell, a cousin of Charleena Lyles, a pregnant mother who was fatally shot by Seattle police in 2017. “We’re all hurting, we’re devastated and we’re mad. But again, we have to look at the overall outcome of this.”

Lawyers for the three Tacoma officers said their clients acted in good faith and were relieved by the verdict. The Pierce county medical examiner ruled Ellis’s death a homicide caused by oxygen deprivation, but the defense argued at trial that methamphetamine in his system and a heart irregularity were to blame.

Anne Bremner, who represented Rankine, said dropping the malice requirement for prosecution was generally viewed as unwelcome by law enforcement officers because of the potential for exposure to criminal liability.

State senator Yasmin Trudeau, a Tacoma Democrat who previously worked in the attorney general’s office on the implementation of I-940, said the law on its own does not guarantee accountability but rather “the opportunity to make the case for accountability” in court.

The measure has since been bolstered, she said, by 2021 laws creating an independent state office to review cases involving police use of deadly force and banning chokeholds and neck restraints.