TOKYO (AP) — Japanese investigators have determined that a collision during a nighttime drill caused the deadly crash last month of two Japanese navy SH-60K Seahawk helicopters, the country’s defense minister said Thursday.

That is according to preliminary flight data analysis, said Defense Minister Minoru Kihara, adding that what exactly led to the collision is still being investigated.

Kihara ruled out mechanical failure of the SH-60K Seahawks from the Maritime Self Defense Force and announced that the exercises, which were halted following the crash, would resume on Friday.

The two SH-60K reconnaissance helicopters carrying a total of eight crew lost contact late on April 20 during nighttime training east of the island of Torishima in the Pacific Ocean, about 600 kilometers (370 miles) south of the capital, Tokyo.

One of the total of eight crew on the two helicopters was found in the waters and later pronounced dead, and the search for the seven missing, as well as the aircrafts’ fuselages, has continued since then.