Seoul (CNN) -- South Korea has returned home four North Korean fishermen rescued from two sinking boats in the Yellow Sea near the maritime border, the semiofficial Yonhap News Agency reported.
The four were found aboard two small boats adrift Thursday in waters just south of the western maritime border near the South Korean island of Baengnyeong, the news agency reported Friday.
Shortly after the fishermen were picked up, South Korea's defense ministry said intelligence officials were investigating to determine whether they intentionally defected. The results of the investigation were not immediately known.
This incident is a departure from recent tensions between the two Koreas along the maritime border.
On Wednesday, South Korea fired at North Korea after it said the country fired three shots close to its maritime border.
No casualties were reported.
Tensions between the neighbors heightened last year after 50 South Koreans were killed in two separate incidents in the area.
In November, the South accused North Korea of shelling Yeonpyeong Island, near the border between the two sides. The incident killed two South Korean marines and two civilians.
North Korea said the shelling was in retaliation for the South's navy firing into Northern waters.
In March 2010, South Korea accused the North of torpedoing and sinking one of its warships, killing 46 sailors.
North Korea has denied attacking the warship Cheonan, though a Seoul-led international investigation found that a North Korean mini-submarine was responsible.