The president wiped away a tear at a ceremony to mark the anniversary of the crackdown in Gwangju, which left more than 165 people dead and over 70 missing and cemented General Chun Doo-Hwan's grip on the country.
"The new government will make greater efforts to reveal the whole picture of the massacre ... To find out who is responsible for issuing the order to open fire on protestors," Moon said in comments broadcast live on television.
As a student, the current president was arrested for leading a protest against the authoritarian rule of the earlier dictator Park Chung-Hee -- the father of Moon's ousted predecessor Park Geun-Hye, many of whose supporters sought to paint the uprising as a communist-inspired rebellion.
"The Moon government stemmed from the Gwangju pro- democracy uprising," the president said.
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More than 10,000 people came to the ceremony at the Gwangju cemetery where victims of the clashes are buried.
Chun seized power in 1980 through a military coup, taking advantage of the vacuum left by the assassination of Park in 1979 by his own spy chief.
Protests then broke out in the southern city.
Chun's martial law troops bludgeoned, stabbed and later shot at protestors during the 10-day long turmoil between May 18 and 27.
Chun was arrested in 1995 after democracy was restored and sentenced to death for his role in the coup and its fallout.