The probe into an unprecedented spate of killings by a suspected serial killer on Cyprus has focused on the toxic waters of a man-made lake on the small Mediterranean island nation, where authorities are scouring for suitcases that may contain the bodies of the killer's victims.

Investigators pulled a suitcase weighed down by a concrete block on Sunday from the murky waters located about 20 miles west of the capital, Nicosia, after the suspect -- a 35-year-old army captain -- told investigators that he put the bodies of three of his victims into suitcases and tossed them into the lake at the abandoned mine complex

Officials said the body recovered in the suitcase on Sunday is believed to be an adult woman who has yet to be identified.

CYPRUS POLICE WIDEN SEARCH AFTER SUSPECTED SERIAL KILLER ADMITS HE MURDERED MORE WOMEN

Fire chief Marcos Trangolas told The Associated Press that visibility is just mere inches in the murky waters, but authorities will continue the search until at least two more suitcases believed to also contain human remains are found.

A second suitcase was detected by a robotic camera, but low visibility, combined with highly acidic waters, has made recovery operations difficult. Sonar equipment from the U.S. is expected to be brought in by Friday and will be deployed by the weekend, In-Cyprus reported.

On Thursday, the 35-year-old suspect, who was not named because he hasn’t been formally charged, told investigators that he had killed four more people than he had previously admitted to.

The victims — all foreigners— include Maricar Valtez Arquiola, 31, from the Philippines; Florentina Bunea, 36, from Romania; and Bunea's 8-year-old daughter, Elena Natalia. Arquiola has been missing since December 2017, while the mother and daughter vanished in September 2016.

Only one victim has been positively identified — 38-year-old Mary Rose Tiburcio, whose bound body was found April 14 down a flooded shaft that was part of the same copper mine.

Her death sparked an investigation that led to the suspect because of his exchange of online messages with Tiburcio, with whom he had a six-month relationship. Tiburcio's 6-year-old daughter, Sierra, is also believed to be among the victims and authorities are searching for her body in a reservoir about 7.5 miles west of the man-made lake.

CYPRUS FEARS POSSIBLE SERIAL KILLER AFTER BODIES OF TWO WOMEN ARE DISCOVERED IN MINESHAFT

Investigators told a court that, during his arrest, the suspect tried but failed to swallow a mobile telephone SIM card.

The suspect on Thursday also pointed investigators to a military firing range, where they discovered another unidentified body. According to the suspect, the body belongs to a woman of either Nepalese or Indian descent.

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The murders have sparked outrage in Cyprus and spurred local officials to bring in British police experts -- including a pathologist and psychologist -- to assist authorities.

Police Chief Zacharias Chrysostomou also has ordered an internal probe to determine whether investigators had failed to properly investigate the women's disappearances, amid public criticism that authorities did little to search for the women when they were reported missing. The island nation has 80 unsolved missing person cases, going back to 1990.

Fox News' Barnini Chakraborty and the Associated Press contributed to this report.