At least two Americans are among the estimated 16 people missing after a cruise ship hit a reef on Friday night and capsized off the west coast of Italy with 4,200 passengers and crew aboard.

Six passengers have been confirmed dead since the Costa Concordia ran aground near Tuscany, tearing a 160-foot hole in its hull and causing the ship to sink. The captain, Francesco Schettino, 52, blames the disaster on rocks not on his navigational charts, but has been arrested on suspicion of manslaughter.

In an emotional news conference, Costa Cruises boss Pier Luigi Foschi tearfully apologized and said, “The company will be close to the captain and will provide him with all the necessary assistance, but we need to acknowledge the facts and we cannot deny human error.”

Foschi said the ship’s route was entered correctly and that “the fact that it left from this course is due solely to a maneuver by the commander that was unapproved, unauthorized and unknown to Costa… He wanted to show the ship, to [go] nearby this island of Giglio, so he decided to change the course of the ship to go closer to the island.”

Survivors described the chaotic scene and said crew members were unprepared for an emergency situation, forcing many passengers to jump overboard and swim for land when the angle of the listing ship made boarding life boats impossible.

In addition, environmentalists worry that the ship could break up and leak its 2,300 tons of fuel. Italian Environment Minister Corrado Clini said his government would declare a state of emergency to release extra funding to avoid such a spill.