JINDO, South Korea, April 21 -- Four more crew members of the South Korean ferry that capsized and sank off the country's southwestern coast last Wednesday were arrested on Monday over negligence of duty and violation of rescue acts charges.

Prosecutors have arrested four crew members, including two first mates, one second mate and one chief engineer, of the ferry Sewol, which was upturned and sank off Jindo Island, a southwestern tip of South Korea.

After being questioned as witnesses, they were arrested Monday as prosecutors need to hold them in custody for questioning for up to 30 days.

Among those arrested was the first mate surnamed Kang who communicated with the Jindo vessel traffic center at the last minutes before the ship sank.

Communication logs, unveiled Sunday by the pan-government response center, showed the vessel's captain may have escaped from the sinking vessel after handing over the communication duty to the first mate.

Some arrested sailors have reportedly told prosecutors that orders of leaving the vessel were not delivered to passengers, which is contradictory to the captain's testimonies.

The arrest came two days after three crew members, including the captain, were arrested for deserting the ship without making efforts to evacuate passengers.

Captain Lee Joon-seok, 69, was arrested Saturday over five charges including negligence of duty and abandonment resulting in death. He ordered passengers to stay put when he and other crew members left the sinking ferry. The captain was among the first to escape from the vessel.

Two sailors, including the third mate and the helmsman, were also arrested for similar charges with Lee. The captain wasn't at the helm when the ship was sinking as the wheel was handed over to the 25-year-old third mate who steered the ship in the rough waters for the first time. The sailor had only six months of experience for the Incheon-Jeju route.

Prosecutors also summoned the ship's regular captain surnamed Shin, who had been on leave when the disaster occurred, as a witness. Shin was known to have told prosecutors that sailing became tough since the sunken ferry had been modified to expand passenger cabins.

Around 40 officials of the ship's operator Chonghaejin Marine have been prevented from leaving South Korea for questioning by prosecutors.

Prosecutors called management officials of the port, from which the ferry departed, to probe into whether the ship was overloaded. Illegal modification and overloading are suspected to have played a role in capsizing the ship. The ferry set sail around two and a half hours later than scheduled due to a heavy fog.

As of Monday morning, the death toll in the accident rose to 64 as divers kept entering inside the hull of the submerged vessel while ships were scouring the waters, with 238 others still missing. The number of the rescued remained unchanged at 174.

The 6,825-ton ferry departed from Incheon Tuesday night for the southern resort island of Jeju, carrying 476 people including 325 high school students and 15 teachers on their way for a four-day field trip.

President Park Geun-hye strongly criticized those responsible for the accident, saying all those responsible will be identified and punished. "What captain and some crew members did was like an act of murder that cannot be acceptable and permissible from a common-sense standpoint," Park said.

Park said that everybody responsible for the disaster "will have to take criminal and civil responsibilities regardless of ranks." Those responsible should be disclosed stage by stage, including those who caused the accident as they violated rules and downplayed safety manuals, those who neglected duty when the ship was sinking and those who acquiesced in irregularities and wrongdoings, she added.

The president has instructed prosecutors to investigate into all processes from how the ship was imported and given a license, to whether the ship was modified, how often safety checks were conducted and how the sailing was approved.