By Andy Pasztor and Jon Emont

A communication breakdown between pilots and air-traffic controllers has emerged as an early focus of the investigation into last weekend's crash of a 1990s-era Boeing 737 in Indonesia, according to people with knowledge of the probe.

The cockpit crew of the Sriwijaya Air jet, which plunged into the Java Sea minutes after takeoff, failed to acknowledge or respond to two radio transmissions from controllers questioning why the aircraft had shifted from its designated route during its climb away from Jakarta's Soekarno-Hatta International Airport, the people said. Instead of flying northeast as expected, the plane veered northwest and at one point, a controller instructed the pilots to execute a turn to get back on track, one of these people said.

It is too early to draw definitive conclusions about the sequence of events before the crash that killed all 62 people on board, according to these people and safety experts not connected with the investigation. Indonesian authorities said the aircraft's flight-data recorder, one of its so-called black boxes, was recovered by divers Tuesday and taken to a laboratory so its contents can be downloaded.

Authorities said they believe the location of the cockpit-voice recorder--which is the other black box--also has been identified, but rough seas prevented retrieval Wednesday.

For now, according to the safety experts, the crew's lack of response on the radio could indicate pilot confusion or distraction. One possibility is that a problem with some mechanical or flight-control system could have consumed the pilots' attention, they said.

Preliminary traffic-control information, the experts said, also raises the possibility that the 26-year old Boeing 737-500, which had been delayed for takeoff due to a storm, could have ended up flying through unexpected weather conditions. The jet quickly plunged more than 10,000 feet with its engines running, but there was no mayday call or emergency transmission from the cockpit, according to investigators and publicly available radar data. Experts said the descent appeared to be a near-vertical dive.

Barring damage to the recorders, data downloaded from them could, within days, provide direction for investigators by revealing details about changes in the jet's speed or trajectory, pilot actions, flight-control commands and potential system malfunctions.

Indonesia's transportation ministry said earlier that air-traffic controllers tried to communicate with the pilots to get the plane back on its designated course seconds before it disappeared from radar. Haryo Satmiko, deputy head of the national transportation safety committee, said via text message that air-traffic control tried to contact Sriwijaya Air Flight 182 more than once to ask why it unexpectedly changed direction.

Others familiar with the probe said the crew failed to respond to a second radio call asking about the plane's direction. That transmission came around the time the jet went into its fatal dive, one of the people said. The exact timing can't be determined until data from the recorders is downloaded.

Another person familiar with the probe said there was a further unsuccessful attempt to make radio contact with the plane after its disappearance from the radar, after which air-traffic control contacted other planes flying in the area to look for and try to contact Sriwijaya Air Flight 182.

Initial inspections of the jet's maintenance records haven't revealed irregularities, according to this person, although authorities will probe further.

Indonesian investigators haven't indicated what the focus of their investigation is. A Boeing Co. spokesman declined to comment.

The U.S. National Transportation Safety Board, which is participating in the probe, is sending four officials to Indonesia, including operational and humanfactors experts. Boeing, engine maker General Electric Co. and the Federal Aviation Administration are assisting the U.S. safety board. A safety board spokesman wasn't immediately available for comment.

Flight 182's takeoff roll, lift off and early climb appeared normal, according to some of the people familiar with the probe. Despite a delay due to stormy weather, publicly available radar data indicates other jets seemingly departed without problems before and after the jet.

While waiting to download data from the recorders, investigators are examining maintenance records, dissecting air-traffic control transmissions and interviewing pilots and mechanics who flew and worked on the Boeing 737-500 before its last flight.

Andrew Tangel and Viriya Singgih contributed to this article.

Write to Andy Pasztor at andy.pasztor@wsj.com and Jon Emont at jonathan.emont@wsj.com

(END) Dow Jones Newswires

01-14-21 0330ET