"The explosion was caused by a mortar attack," Czerwinski told a news conference on Wednesday in Iraq, broadcast on Polish private television station TVN24.
Earlier in the day, ministers from Poland and Latvia had said the blast on Tuesday had "probably" been caused by a mortar attack.
Officials had initially blamed the deaths of three Slovakian, two Polish and one Latvian soldier on an accident that occurred during a de-mining operation at an old munitions depot dating back to the Saddam Hussein era.
The deaths were the first casualties among troops from Latvia and Slovakia and brought to six the number of Polish deaths. Four Polish civilians, among them a journalist and a cameraman, have also been killed in Iraq.
Poland commands a multinational force of some 6,500 troops, including 2,400 of its own soldiers, in the south-central part of Iraq.
Latvia and Slovakia currently have 116 and around 100 soldiers in Iraq respectively.
A special military committee of inquiry was sent by Poland to Iraq on Wednesday to examine the causes of the explosion.