A Yemeni court ordered an investigation into former President Ali Abdullah Saleh and 11 of his aides in connection with the March 2011 attack on anti-government protesters.
Yemen was one of the countries swept up in the so-called Arab Spring, a wave of uprisings that brought down the governments of Tunisia, Egypt and Libya. Human Rights Watch said plainclothes gunmen fired on Yemeni protester, leading to 45 deaths and about 200 injuries.
Director of Middle East programs for Human Rights Watch Sarah Leah Whitson said Yemen has an opportunity to address some of the issues surrounding Saleh's legacy.
"The authorities should leave no stone unturned until they have identified the people responsible for these killings and held them to account," she said in a statement.
Saleh served as president for more than 30 years. Vice President Abdu Rabbo Mansour Hadi, took over following a political deal reached in February 2012. That deal gives Saleh amnesty but not in the international courts.
A reconciliation campaign was launched last month in Yemen. The International Committee of the Red Cross said it was struggling to meet the needs of a population left vulnerable to social unrest and political tensions.