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SEPANG, Malaysia – A signaling system was disabled on the missing Malaysia Airlines jet before a pilot spoke to air traffic control without mentioning trouble, a senior Malaysian official said Sunday, reinforcing theories that one of the pilots may have been involved in diverting the plane and adding urgency to the investigation of their pasts and possible motivations.

With the increasing likelihood that Flight 370 was purposefully diverted and flown possibly thousands of miles from its planned route, Malaysian officials faced more questions about how the investigation, marked by days of contradictory government statements, might have ballooned into a global goose chase for information.

Prime Minister Najib Razak acknowledged Saturday that military radar and satellite data raised the possibility that the plane could have ended up somewhere in Indonesia, the southern Indian Ocean, or along a vast arc of territory from northern Laos across western China to Central Asia. Malaysian officials said they were scrambling to coordinate a 25-nation effort to find the plane.

And on Sunday, Malaysia’s defense minister added a critical detail about investigators’ understanding of what transpired in the cockpit in the 40 minutes of flight time before ground controllers lost contact with the jet. The determination that the last verbal message to the control tower – “All right, good night,” someone said – came after a key signaling system had stopped transmitting, perhaps having been shut off, appeared likely to refocus scrutiny on the plane’s veteran pilot, Capt. Zaharie Ahmad Shah, and his young first officer, Fariq Abdul Hamid.

Commercial passenger planes use radio or satellite signals to send data through ACARS, the Aircraft Communications Addressing and Reporting System. The system can monitor engines and other equipment for problems that may need attention when a plane lands.

Although officials had already said that ACARS was disabled on the missing plane, it had previously been unclear whether the system stopped functioning before or after the captain radioed his last, brief words to Kuala Lumpur, in which he did not indicate that anything was wrong with the signaling system or the plane as a whole.

During a news conference Sunday, the defense minister, Hishammuddin Hussein, who is also acting minister of transportation, gave a terse answer: “Yes, it was disabled before.”

The fate of the plane and the people it carried has become a formidable riddle, bringing together questions about aviation technology, investigation of the private lives of passengers and crew, and a search across a vast arc of the Indian Ocean and often rugged, remote terrain in Asia, with no clear idea of where to begin.

“It’s something of the scope I’ve never seen before,” Cmdr. William Marks, the spokesman for the U.S. Navy 7th Fleet, which sent two guided-missile destroyers to join the search, said in a telephone interview. Of the size of the Indian Ocean, he said: “Essentially, it’s like looking for a person somewhere between New York and California. It’s that big.”

The Malaysian authorities trying to locate Flight 370 have not singled out the pilots or crew as the only potential suspects. Officials said Sunday that they would scrutinize the backgrounds of all 239 passengers and crew onboard, as well as ground crew and engineers who worked on the Boeing 777 jet, which took off at 12:41 a.m. local time on March 8.

“The Malaysian authorities are refocusing their investigation on all crew and passengers,” Hishammuddin said. “I understand the hunger for new details, but we do not want to jump to conclusions.”

According to the airline, he said, “the pilot and co-pilot did not ask to fly together on MH370.” If true, that point might undermine speculation that the two men acted in unison in the plane’s disappearance.

Hishammuddin confirmed that the Malaysian police had searched the Kuala Lumpur homes of the captain and co-pilot Saturday. The police took to their offices a flight simulator that the pilot, Zaharie, had kept at his home, and reassembled it so that experts could examine its workings, Khalid Abu Bakar, the inspector general of the Malaysian police, told reporters.

Rohan Gunaratna, a professor at Nanyang Technological University in Singapore who studies security and terrorism in Asia, said that while the weight of suspicion would inevitably fall on the pilots and other crew members, investigators were following established procedure by examining everyone on the missing plane.

Soon after the plane disappeared, FBI agents and other U.S. investigators “scrubbed” the names of the pilots and passengers, including two Iranian men who traveled on stolen passports, to determine whether they had any connections to terrorists. They have found no such connections, officials said Sunday, while cautioning that the home countries of some of the passengers had not yet supplied full background checks on their citizens who were aboard the plane.

“You can’t rule anything out, so everyone on the plane must be treated as a potential suspect,” Gunaratna said in a telephone interview. He said he had heard no credible information of any militant group’s claiming responsibility for seizing the plane.

“That does not mean the possibility does not exist, but at this stage of the investigation it’s important to be open to all the possibilities,” he said.