Executives of Kogalymavia—a Russian carrier that also goes by the Metrojet brand—have ruled out technical or mechanical failures, while U.S. officials are looking into terrorism claims. Here’s what we know so far about the Airbus A321 crash:

What are the details of the flight?

Metrojet Flight 9268 left the Sharm el-Sheikh International Airport in Egypt at 5:50 AM ET, and was bound for St. Petersburg, Russia. The jet was a charter flight that is usually filled with Russian tourists coming back from a popular Egyptian resort. Twenty-three minutes after takeoff, the plane suddenly experienced a drastic drop in altitude and disappeared from flight radars. According to data released so far, the jet dropped some 6,000 feet in about 22 seconds, and according to sources who spoke to the Wall Street Journal, its speed slowed to 100 mph from 460 mph. The plane was later found to have crashed in a remote mountainous area within Sinai. You can see the change in altitude and flight route here, as provided by Flightradar24.

Who is Metrojet?

Founded in 1993 as a regional carrier, the company is said to own five Airbus planes with an average fleet age of 16 years, and operates a large number of flights to Egypt. Some have criticized the airline for a host of operational issues, including failing to pay its staff for over two months, and a previous accident in January, 2011 when a fire started on a Tupolev Tu-154B-2 jet while it was taxiing on the runway.

What’s the history behind this plane?

This Airbus A321 was about 18 years old and had clocked around 21,000 flights, according to the Flight Safety Foundation’s Aviation Safety Network. In the estimation of flight expert Clive Irving, that’s a relatively low number for present-day budget carriers. What is most interesting about the plane, Irving notes, is a previous accident in 2001 while the plane was operated by Middle-Eastern Airlines. The aircraft’s tail end struck the tarmac in a botched landing, and if later repairs weren’t comprehensive, Irving believes this could have led to a compromised section of the plane during last weekend’s crash.

Why does the tail-end section of the Metrojet plane matter?

That’s the part that has given investigators the most significant clue into what caused the plane’s crash. Debris from the tail section was found in a separate area from the rest of the fuselage, suggesting that the two sections might have been separated mid-flight. This is the same theory that Kogalymavia executives are using to say that “mechanical impact” was the overriding cause of the crash, vs. pilot or technical errors. (Russian authorities have said it’s too early to speculate.)

What else could have caused the plane to crash?

A missile-strike by ISIS—according to ISIS. The terror group has claimed responsibility for the strike and is hinting that it was done in retaliation to Russian airstrikes in Syria that have targeted ISIS. A few weeks earlier, an ISIS spokesman urged a holy war against the Russians for their actions. The claim has been dismissed by aviation experts, however, who say ISIS’s surface-to-air weaponry is unable to hit a plane more than 30,000 feet in the air.

MANPADS wouldn't be able to get close to a jet at 30,000+ feet, their flight ceiling is closer to 10,000 feet https://t.co/e3cJOREP0E

— Eliot Higgins (@EliotHiggins) October 31, 2015

Russian and Egyptian transport authorities have also dismissed ISIS’s claims, but their U.S. counterparts aren’t ruling it out. The Sinai province has been a battleground between ISIS-affiliated militants and Egyptian security forces over the last few years, and the Metrojet crash bears some similarities to Malaysia Airlines Flight MH17, which many say was shot down by pro-Russian insurgents over Ukraine in July, 2014. A year ago, the European Aviation Safety Agency and Federal Aviation Administration issued a warning to airlines not to fly over the Sinai region.

What’s next?

Four airlines—Emirates, Qatar Airways, Air France, and Lufthansa—are diverting their planes away from the region in response to the tragedy. Investigations will begin in earnest, while others are speculating that a bomb or internal sabotage might have brought down the Metrojet plane. The aircraft’s so-called black box has been retrieved and sent for analysis, and Russian investigators are hoping communication recordings will provide more clues into the worst Airbus accident in a decade.