LODI, Calif. (AP) — FBI and other federal agents this week seized credit card receipts, skydiving release forms and videos of people skydiving from a skydiving center in central California that has recently seen several deaths, the owner said Thursday.

Lodi Parachute Center owner Bill Dause said at least 15 federal, state and local agents spent about 10 hours Tuesday going through his store in Lodi but that they did not give him a reason for the search.

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“All they said was that it was an ongoing investigation,” Dause said.

Gina Swankie, spokeswoman for the Federal Bureau of Investigation in Sacramento, confirmed a search warrant was served at the skydiving address but said she could not provide any other information.

Four skydivers who used the Lodi Parachute Center as a starting point have died since 2016. Dause said the deaths were investigated and authorities found no violations.

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In September 2016, 20 instructors at the center were suspended and 120 others were told they needed to undergo more training after an investigation by the United States Parachute Association.

A review of news stories going back to 1981 found at least 20 people have died flying out of Lodi Parachute Center, the Sacramento Bee reported.

“Skydiving is a high-risk activity,” Dause told KCRA-TV. “None of the fatalities we’ve had out here were interrelated. It’s not like everybody had a bad parachute that was packed by someone here or the parachute was owned by the Parachute Center.”

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Dause said the center was open again on Wednesday.