GALVESTON — Twenty bottlenose dolphins were found dead on beaches in Galveston and Jefferson counties on Monday, rekindling a mystery officials said may never be solved.

Officials said another dolphin was discovered Tuesday, bringing the total to 21.

Monday’s discovery on the Bolivar Peninsula follows an unusually high number of dolphin deaths about the same time last year but may provide just as little in the way of answers.

Almost 70 dolphins washed up on Galveston County shores between Feb. 27 and March 23, 2007.

Those dolphins were badly decomposed, hampering efforts to test tissue samples. Of those tested, the results were either inconclusive or showed no abnormalities, said Blair Mase, who coordinates regional efforts involving stranded marine animals for the National Oceanographic and Atmospheric Administration.

The dolphins found Monday are also badly decomposed, so drawing conclusions from tissue samples could again prove difficult, Mase said. Members of the Texas Marine Mammal Stranding Network were collecting samples Monday.

Mase did not know exactly where the dolphins were found or who reported them. Efforts to reach members of the stranding network were unsuccessful.

Twelve dead dolphins were reported Monday morning. Six were newborns, three were yearlings and three were elderly, said Kim Amendola, a spokeswoman for NOAA’s National Marine Fisheries Service. Eight other dolphins were found later in the day.

The newborns and elderly “are the most vulnerable of the population that are succumbing to whatever’s going on out there,” Mase said.

Texas has a healthy population of bottlenose dolphins, and it’s common to have deaths in calving season during February and March, Mase said. But finding 20 dolphins on the beach in one day, or 70 in one season, is unusual, she said.

In 2007, the animals washed up along the Galveston seawall area and in Crystal Beach, High Island, Sabine Pass, Surfside Beach and Jamaica Beach.

Dolphins also washed up on shores at a parish in Louisiana, prompting NOAA to investigate the situation in all three jurisdictions as an “unusual mortality event.”

Since 1991, NOAA has classified 41 episodes of stranded marine mammals as unusual mortality events.

Scientists determined causes for some of the events, including diseases, harmful algae blooms that produce toxins and environmental conditions such as El Niño, a periodic change in conditions in the tropical Pacific Ocean that can affect weather worldwide.