A set of human remains that were discovered at Lake Mead have been identified as a man who disappeared in 1998.

The Clark County Medical Examiner’s Office said Thursday the remains were found on three separate dates last summer — July 25, Aug. 6 and Aug. 16 — and belonged to Claude Russell Pensinger, a 52-year-old Las Vegas man who disappeared in 1998. However, his cause of death has not been determined.

Pensinger disappeared on or around July 20, 1998 while fishing on the reservoir, according to the Las Vegas Review-Journal. His empty boat was found running in circles.

Pensinger’s remains were found at Boulder Beach on the Nevada side near the Hoover Dam.

A number of other sets of human remains have been found recently as the water levels at Lake Mead continue to drop at an unprecedented rate. At least two other sets of remains have been identified, but their causes of death have not been determined. The remains of a third man, who was found in a 50-gallon drum and appeared to have died by gunshot wound in the 1970s or ‘80s, have not been identified.

Lake Mead, which was created by the construction of the Hoover Dam in 1935, is mostly filled by the Colorado River. However, a 20-year-drought has been drying up the reservoir. Earlier this month, the Interior Department said the Colorado River Basin was likely to see “unprecedented water shortages.”

In an attempt to slow the lake’s decline, additional water from the Colorado River was sent into the reservoir through Lake Powell and the Glen Canyon Dam in Arizona earlier this week. The water will flow through the Grand Canyon before it settles in Lake Mead.