A United Nations report released in December blamed the then-leader of the country's military government, Captain Moussa Dadis Camara, for the massacre.
The massacre occurred as opposition supporters staged a rally amid concerns that Camara - who seized power in 2008 after the death of Lansana Conte, the president - was planning to renege on a pledge to hold civilian elections.
Besides scores who died after soldiers opened fire in the city's main sports stadium, more than 100 women were raped during the incident.
Bensouda said "atrocious crimes" had been committed on September 28 as "men in uniform attacked civilians, they killed and wounded".
"In full daylight they mistreated, violated and submitted women to unprecedented sexual violence."
Despite her conclusions, Bensouda said Guinea could become an example if it was willing to bring the main perpetrators to justice.
"These few days working in Guinea confirmed that Guinean institutions and the ICC can work in a complementary way: either Guinean authorities can prosecute the main people in charge themselves, or they will turn to the court to do it," she said.
However, a commission appointed by the country's military rulers has already absolved Camara of responsibility.
Since the massacre, Camara has faced an attempt on his life and is recovering in Burkina Faso, while a transitional government has been appointed to steer the country back to civilian rule.