In a heartbreaking mass stranding event, nearly 200 dolphins washed ashore in the Cape Verde Islands off West Africa this week. Now, experts in Spain are trying to determine what caused the majority of those marine mammals to die.

According to BIOS Cape Verde, an environmental association in the area, 163 melon-headed dolphins were found Tuesday on a beach on Boa Vista Island. 100 officials, residents and tourists tried to drag some of the living dolphins back out to sea, but they were largely unsuccessful.

In the end, authorities buried 136 dead dolphins using bulldozers.

Experts from BIOS collected samples from 50 animals and are studying them to find the root cause of the mass die-off event. They also collected four of the dolphins and froze them in their facilities to be studied, the organization said on Facebook.

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Spanish scientists from the University of Las Palmas de Gran Canaria are traveling to Boa Vista to perform necropsies and train locals for future incidents.

The issue isn't isolated to the Cape Verde Islands. Almost 300 dead and dying dolphins have washed ashore in the Gulf Coast since February — about three times the usual number.

Puzzled scientists in the region are exploring several possible causes, from the lingering effects of the oil spill to skin lesions on many recovered dolphins that indicate freshwater exposure. It could also be chemicals, pollutants or a combination of the two.