Kim, 69, died at 8:30 a.m. Saturday, according to state media.
A tearful broadcaster reported that Kim died due to "overwork" after "dedicating his life to the people."
Kim suffered "great mental and physical strain" while on a train during a "field guidance tour," North Korea's state-run KCNA news agency reported.
A look back at the life of Kim Jong Il
More specifically, the agency reported that Kim suffered a heart attack and couldn't be saved despite the use of "every possible first-aid measure."
He had been treated for "cardiac and cerebrovascular diseases for a long period," KCNA noted.
