(CNN) -- The South Korean ferry disaster that killed more than 260 people last month was caused in part by excessive cargo and a failure to tie that cargo down properly, the joint police and prosecuting team investigating the disaster said Tuesday.
It marked the first time South Korean investigators said what they believe led to the April 16 sinking of the ferry Sewol, which was carrying 467 passengers and crew -- including more than 300 high school students on a field trip -- when it capsized.
Investigators said they've indicted four employees of the ferry's owner, Cheonghaejin Marine Co., in the last two weeks, including a senior executive Tuesday. Details about the charges weren't immediately available.
Authorities took aim at the cargo Tuesday, saying its weight was more than double the ship's limit.
The cargo wasn't tied properly -- and the loosely tied goods helped cause the ship to capsize, senior prosecutor Yang Joong-jin said. "The lashing devices that should have held cargo goods steady were loose, and some of the crew members did not even know" how to use them correctly, Yang said.
Investigators had been probing the possibility the ship overturned because cargo shifted and forced the ship off balance.
At least 269 people died in the disaster, which happened while the ferry was traveling from Incheon to the resort island of Jeju, off South Korea's southwestern coast. Thirty-five people still are unaccounted for, according to the country's coast guard.