Malaysia Airlines Flight 17 was cruising just 1,000 feet above restricted airspace when it was struck by a missile in Ukraine's Donetsk region, according to aviation and intelligence officials.
Despite ongoing violence in eastern Ukraine, including the recent downing of two military aircraft, Ukrainian aviation officials had closed the region's airspace only below 32,000 feet in altitude.
The Boeing 777, en route from Amsterdam to Kuala Lumpur, the Malaysian capital, with 298 people aboard, was at 33,000 feet when it vanished from radar screens, according to
European aviation authorities.
"No dispatcher would have sent an aircraft into this region if they thought there was a risk it might not come back," said Sean Cassidy, vice president of the Air Line Pilots Assn. "But there is no uniform rule that's out there that forbids the aircraft from entering that airspace."
Until recently, the battle in eastern Ukraine was largely ground-based and not thought to involve the sort of sophisticated weaponry that threatened high-altitude commercial flights.
"For many onlookers, the situation in Ukraine was thought to be little more than a riot that had gotten out of control," said John Pike, director of GlobalSecurity.org, a website for military policy. "You don't go re-routing planes every time people on the ground get upset."
In recent days, though, military aircraft were downed by antiaircraft munitions, which should have concerned aviation authorities, he said.
"Nobody thought it was going to be like this," Pike said, noting that shoulder-fired, man-portable air-defense systems, known as MANPADS, could not reach such altitude.
Since the Malaysian plane went down, the Ukrainian authorities have closed all routes in the country's east.
"All flight plans that are filed using these routes are now being rejected," according to Eurocontrol, the European air navigation service. "The routes will remain closed until further notice."
The Federal Aviation Administration had prohibited U.S. carriers from airspace over Ukraine's Crimea region since April. On Thursday it expanded the prohibition to include eastern Ukraine.
U.S. intelligence sources have confirmed that the 777 was downed by a ground-to-air missile over territory controlled by pro-Russia militants.
Most commercial jets are not outfitted with missile warning and defense systems, which are standard on many military aircraft. So it's unlikely that the Malaysia Airlines pilot would have been aware that the missile's guidance system had locked onto the aircraft.
"As of now, only a few aircraft assigned to transport government officials have these systems," said Brad Curran, aerospace and defense analyst at consulting firm Frost & Sullivan. "Defending against a comprehensive air defense system fielded by a nation-state, equipped with a variety of radars, missile guidance systems, and long-range sophisticated missiles, has not been considered."
Commercial jets have been accidentally shot down before. A Soviet fighter plane shot down Korean Air Lines Flight 007, carrying 269 people, including a U.S. congressman, in 1983. Five years later, a U.S. Navy warship mistakenly shot down Iran Air Flight 655, carrying 290 people.
But the examples are few and far between.
The 777, a jet flown by nearly all of the world's major airlines, is considered one of the safest aircraft in the worldwide fleet.