KUALA LUMPUR, Malaysia — The Malaysia Airlines jetliner that went down in war-torn Ukraine did not make any distress call, Malaysia’s prime minister said Friday, adding that its flight route had been declared safe by the global civil aviation body.

Najib Razak, who addressed a middle-of-the-night news conference after speaking with leaders of Ukraine and the Netherlands, and to President Barack Obama, said “no stone will be left unturned” in finding out what happened to Flight 17 and the 298 people on board.

It is the second tragedy to hit Malaysia Airlines this year. Its Flight 370 disappeared March 8 while en route from Kuala Lumpur to Beijing. It has not been found, but the search has been concentrated in the Indian Ocean west of Australia.

“This is a tragic day in what has already been a tragic year for Malaysia,” Najib said.

In both tragedies, the planes were the wide-bodied Boeing 777-200.

Najib said that Ukrainian authorities believe Flight 17, which was on its way from Amsterdam to Kuala Lumpur with 283 passengers and 15 crew, was shot down Thursday. Among the passengers were three infants.

A U.S. official said American intelligence authorities believe a surface-to-air missile took down the plane, but it is not clear who fired it. He said it appears unlikely the Ukrainian government, which has denied responsibility, shot down the plane because it doesn’t have the capabilities. Pro-Russia separatists fighting the government have also denied any responsibility.

“At this stage, however, Malaysia is unable to verify the cause of this tragedy but we must, and we will, find out precisely what happened to this flight,” Najib said. “If it transpires that the plane was indeed shot down we insist that the perpetrators must swiftly be brought to justice,” he said.

At least 27 of the victims were Australian, and Prime Minister Tony Abbott said Friday that authorities owed it to the families of the dead to find out what happened.

“As things stand, this looks less like an accident than a crime. And if so, the perpetrators must be brought to justice,” Abbott told parliament.

Abbott said he thought Russia was behind the shooting down of the plane.

“This is a grim day for our country and it’s a grim day for our world,” he said. “Malaysia Airlines MH17 has been shot down over the eastern Ukraine it seems by Russian-backed rebels.”

Najib said the aircraft flight route was declared safe by the International Civil Aviation Organization. The International Air Transportation Association had also stated that the air space that the aircraft was traversing was not subject to restrictions, he said. Besides, “Malaysia Airlines has confirmed that the aircraft did not make a distress call.”

Still, a former head of airports security group BAA suggested that many airlines including Malaysia Airlines had continued to use the route despite warnings because it was shorter and cheaper.

“It is a busy aviation route and there have been suggestions that a notice was given to aviators telling airlines to avoid that particular area,” said Norman Shanks, who is a professor of aviation security at Coventry University in England.

“But Malaysia Airlines, like a number of other carriers, have been continuing to use it because it is a shorter route, which means less fuel and therefore less money,” he told The Associated Press.

Hours after the disaster, Malaysia Airlines announced all European flights will henceforth take an alternative route.

Najib said the Ukrainian government has promised a full and thorough investigation which will include Malaysian officials. He said they will also negotiate with rebels to “establish a humanitarian corridor to the crash site.”

In his conversation with Obama, Najib said they agreed that “the investigation must not be hindered in any way. An international team must have full access to the crash site. And no one must interfere with the area, or move any debris, including the black box.”

Earlier, several relatives of those on board the Malaysian airliner began arriving at the Kuala Lumpur International Airport to seek news of their loved ones.

A distraught Akmar Mohamad Noor said her older sister, who lives in Geneva, was on her way back to celebrate Eid with the family.

The 67-year-old sister has lived in Geneva for 30 years and last visited the family in Kuala Lumpur five years ago, she said.

“She was coming back from Geneva to celebrate (Eid) with us for the first time in 30 years,” Akmar said in between sobs. “She called me just before she boarded the plane and said ‘see you soon,” Akmar said.

She said the family saw the news on TV and rushed to the airport to get details.

Several other angry relatives were shouting and demanding to see the passenger manifest but there was no official from Malaysian Airline present, and security guards prevented them from going into the airline’s operating area.

“We have been waiting for four hours. We found out the news from international media. The Facebook is more efficient than MAS. It’s so funny, they are a laughing stock,” an angry young man told reporters. He declined to give his name.