SEOUL, May 15 -- An abrupt turn off the course was the main cause of the South Korean ferry's sinking, local broadcaster YTN reported citing a joint investigation team of police and prosecutors on Thursday.

The passenger ship Sewol, which capsized and sank off Jindo Island, a southwestern tip of South Korea, on April 16, was illegally modified by the ship's operator Chonghaejin Marine to expand passenger cabins, worsening the vessel's capability to recover a balance.

Waters lacked in the ballast tanks to overload cargo, contributing to the worsening of the recovering ability. The ferry was loaded with 2,142 tons of freight more than admitted. More than 1,300 tons of ballaster water was drawn off for the overloading.

Under the unstable condition, an inexperienced sailor made a sudden turn at more than 15 degrees in the swift currents. The area is notorious for the country's second-fastest tidal currents.

When the ship began tilting heavily to one side, the captain was not at the helm and the wheel was handed over to the third mate who controlled the ship in the notorious waters for swift currents for the first time.

Prosecutors indicted the captain, chief engineer, first and second mates of the ferry for several charges, including murder, attempted murder and violation of the maritime rescue law.

Those crew members have been harshly criticized for abandoning the ferry after ordering passengers, mostly high school students, to stay where they were. The 68-year-old captain Lee Joon-seok was among the first rescued.

Other 11 survived sailors were charged with causing death by negligence and violation of the maritime rescue law, which could lead to as long as 45 years in prison.

On the 30th day into search, 284 people have been confirmed dead, and 20 others remain missing. No survivors have been reported since 172 people were saved from the sea and ship on the day the ship sank.

The first stage search operations ended on Thursday. Coast Guard chief Kim Seok-kyun said in a statement earlier that the first stage search focused on 64 passenger cabins, noting future search operation will focus on the third and fourth floors of the five-story vessel where the 20 missing people are believed to be trapped.

Affiliates of the Chonghaejin Marine and Yoo Byung-eon, the de facto owner of the ship operator, and his families were suspected of borrowing massive loans illegally, the country's financial watchdog said.

The Financial Supervisory Service (FSS) has probed into suspicions that Yoo family, the ship operator and its units borrowed illegal loans from some financial institutions since the day the ferry sank.

Among the 70 affiliates relating to the Chonghaejin Marine were 46 companies suspected of illegal transactions with financial companies. Among 186 people connecting with Yoo were 90 people suspected of borrowing illegal loans from financial institutions.

The suspected illegal loans reached 374.7 billion won (around 370 million U.S. dollars), including those to affiliates and those who are close aides to and family members of Yoo.

Those loans were provided after the lax judgment on loans, surpassing the ceiling on loans and failing to impose penalties on inappropriate fulfillment of loan terms.

They also violated the foreign exchange transaction law, while concealing transactions with those linked to Yoo and committing accounting fraud.

Prosecutors have ordered Yoo, 73, to appear at the prosecutors' office, but he has disobeyed the order.

Arrest warrant for Yoo's eldest son was issued Monday after he disobeyed summons. Yoo's second son and eldest daughter, who now live abroad, have been evading investigations.

Yoo and his family members were suspected of setting up paper companies to gather slush funds and illegally transfer money abroad by embezzling corporate funds, which may link closely with the poor safety management of the ship operator.

Chief executive officer of Chonghaejin Marine was arrested last Friday for overlooking the inveterate overloading. Four other executives and staff were arrested earlier on charges of accidental homicide and violation of the ship safety law.

The Board of Audit and Inspection (BAI), the country's state auditor, said it will launch an inspection this week into government ministries and agencies including the Ministry of Oceans and Fisheries, the Ministry of Security and Public Administration and the Coast Guard as well as the Korean Register of Shipping and the Korea Shipping Association, which came under public criticism of lax response to the accident.