LONDON--The first of two black boxes from Malaysia Airlines Flight 17 was successfully downloaded Wednesday with work on the other device due to take place Thursday, the Dutch Air Safety Board said.

The two devices that store data from the aircraft and record conversations in the cockpit arrived Wednesday in the U.K. for analysis by the British Air Accidents Investigation Branch at Farnborough, England. The AAIB is gathering data for the Dutch-led probe into the crash of the Boeing 777 on July 17 that killed all 298 people onboard.

"The Cockpit Voice Recorder data was successfully downloaded and contained valid data from the flight," the Dutch agency said in a statement. The device was damaged, though the critical memory module was intact with no evidence it was tampered with, it said.

"The downloaded data have to be further analyzed and investigated," the group said.

Work on the flight data recorder will take place Thursday. If that information can be extracted the data from both recorders will be combined to gain a fuller understanding of what happened in the shoot-down of the plane while cruising at 33,000 feet above eastern Ukraine.

Analyzing the data isn't expected to help officials determine who shot down the jetliner, though it could rule out any technical failure leading to the accident.

The Dutch Safety Board said earlier Wednesday it could take several weeks to fully complete the data analysis.

Air accident investigators have failed to inspect debris at the crash site because of uncertainty about the security situation there.

Write to Robert Wall at robert.wall@wsj.com

(Corrections & Amplifications--This item was corrected at 1719 GMT because it misstated in the headline the number of black boxes that had been downloaded. The data from the first of two black boxes was successfully downloaded.)

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