Senior prosecutor Chung Soon-shin announces the launch of the probe into Chonghaejin Marine Co., the operator of the ill-fated ferry Sewol on Sunday. (Yonhap)

South Korean authorities widened their investigation into the cause of the ferry disaster on Monday by arresting four crew members and banning others involved from leaving the country.

Prosecutors on Monday barred the family that owns Chonghaejin Marine Co., the operator of the ill-fated ferry Sewol, from leaving Korea.

The Incheon District Prosecutors’ Office took the measure as part of an expanding probe to identify whether the ship’s renovation and the crew members’ lack of safety education played a role in the sinking of the ship.

Up to 44 executives and shareholders of Chonghaejin Marine were banned from leaving the country, including CEO Kim Han-shik, 72, who disappeared from the public eye after a press conference Thursday.

“The measure is to question them and hold them responsible for the poor management of the vessel,” an official from the prosecution told Yonhap.

Prosecutors also barred Chonghaejin Marine’s largest shareholder identified by the surname Yoo from leaving the country.

Investigators suspect that Chonghaejin Marine is indirectly owned by Yoo and his brother, whose father was the owner of the Sewol operator’s parent firm Semo Marine that went bankrupt in 1997.

“We’re looking into the company’s overall management and supervision of the staff,” the prosecution said.

The Korean operator bought the 20-year-old Japanese ferry in 2012 and added another deck during a four-month renovation, raising the total capacity of passengers from 840 to 956.

The new design also added about 239 tons to its original form, raising questions about its possible effect in keeping the boat balanced.

The overloaded cargo and passengers may be the very cause of the Sewol’s capsizing, said Kim Hee-soo, a former prosecutor who led the investigation into another major ferry disaster in 1993.

“There are similarities between the two accidents, including the fact that both ships were overloaded. The 110-ton Seohae ferry in 1993 was apparently thrown off balance after adding another 10 tons of cargo,” Kim said in an interview with a local broadcaster.

If the operator is found responsible for the poor management of the ship and lack of safety drills, the prosecution may widen the probe into state organizations that authorized the ship’s operation and safety certification, local reports said.

The prosecution also issued warrants Monday to detain three mates and the chief engineer of the Sewol on suspicion of abandoning the ship without evacuating the passengers.

The move came after the ship’s captain Lee Joon-seok and two other crew members were earlier charged with negligence of duty and violation of maritime law.

Of the four crew members under arrest, one is the first mate whose voice allegedly appeared in the communications log with the Jindo Vessel Traffic Service disclosed Sunday.

The revealed log implied that the first mate repeatedly asked about the time of the Coast Guard’s arrival, delaying the evacuation order.

“We’re looking into whether the crew members were discussing escape plans without notifying the passengers,” said Ahn Sang-don, a prosecutor on the team.

The intense investigation has also pressured another chief engineer to attempt suicide early Monday, the Mokpo Police Station said.

The chief engineer is now in stable condition at a nearby hospital but will soon be summoned for further questioning, police said.

The captain and the other 14 surviving crew members have been under fierce public criticism for escaping the sinking ferry without helping hundreds of passengers escape.

By Suk Gee-hyun (monicasuk@heraldcorp.com)