A gravel-pit rockslide that killed two people was caused by a seam of clay that gave way after a period of wet weather, a federal agency concluded.

No citations will be issued against the pit operators for violating safety standards, the federal Mine Safety and Health Administration said Wednesday in releasing its report on the accident.

The massive May 19 slide occurred at a Morrill Asphalt Paving Co. sand and gravel pit on the west side of U.S. 97A, about a mile south of Rocky Reach Dam.

The bodies of day foreman Tim Grace, 27, of Spokane, and the 5-year-old son of an employee were found several days after the slide under tons of rock and dirt.

Lloyd Logging of Twisp, Wash., was crushing rock at the site for Morrill. Fred Hanson, the mine safety agency’s western district manager in Vacaville, Calif., said neither company violated mine safety and health standards.

Inspections by two geologists and tests of soil samples led to the conclusion that clay was to blame, Hanson said. An estimated 1.5 million cubic yards of dirt and rock overlying the clay slumped into the pit area.

“The cause was some clay seams underneath that deposit of sand and gravel that were deposited there eons ago,” Hanson said. “Mining the toe of the sand and gravel deposit and wet weather in the spring caused it to slide.”

Seven Lloyd Logging employees ran from danger and survived.

Inspectors had visited the site two weeks before the slide and found no health or safety violations, Hanson said.

A contractor hired by Morrill began crushing rock at the site again on Sept. 11.