Thirty-six men have been executed since Davis died. His sister talks about her fight to clear his name and end the death penalty

The sister of Troy Davis, the man whose execution a year ago amid substantial evidence that he was innocent sparked protests across the US and the world, has vowed to keep on fighting until the death penalty is ended "one state at a time".

Speaking to the Guardian on the first anniversary of her brother's death, Kimberly Davis said that her family was still determined to clear his name of the crime for which he was executed – the 1989 murder of a police officer. Seven of the nine witnesses who testified at the trial recanted their accounts before the state of Georgia killed him by lethal injection.

"They insisted on executing an innocent man despite so much doubt around the case. If those seven witnesses were credible enough to put my brother on death row, then why weren't they credible when they recanted?" Davis said.

Troy Davis was pronounced dead at 11.08pm on 21 September 2011. He was aged 42. Before he died he told Kimberly and other family members "that he wanted us to continue the fight to clear his name and end the death penalty", she said.

Kimberly Davis is following her brother's exhortation. Next month she will be in California campaigning on behalf of the SAFE California Act that proposes to commute the death sentences of all 725 current death row inmates in the state to life without parole.

The proposal is put to California voters as a referendum on 6 November.

"My brother was murdered by the state of Georgia. For the Troy Davises who came before him, and the Troy Davises who will come after him, we want to stop the killing of innocent men," she said.

Despite the extraordinary welling up of outrage around the Davis execution, prisoners have continued to be executed steadily over the past 12 months. The first man to die after Troy Davis, Derrick Mason, was put on the gurney in Alabama just the following day – on 22 September 2011.

The most recent execution took place in Ohio yesterday. Donald Palmer, 47, was killed by lethal injection for a double murder in 1989.

Through the year, deep ethical and procedural problems with the death sentence in the US have been displayed by the cases of the 36 men (they have all been men) who have been executed since Troy Davis. They include issues of mental illness and learning difficulties, childhood abuse and a length served on death row that has been denounced as a form of torture (see side story).

But there have also been strides made during the year towards the withering away of the death penalty. Last November the governor of Oregon, John Kitzhaber, put a moratorium on all executions and in April Connecticut became the fifth state in as many years to abandon capital punishment altogether.

Kimberly Davis said that although her brother's killing generated a great deal of anger around America, she was clear that anger was not the emotion that would win this battle.

"My brother said before he died that he forgave those who testified against him. He said he held no animosity in his heart. He knew that going out and spreading the word was a far more effective weapon than shouting out in anger."