Maria, center, the mother of Alexei Alekseyev, one of the plane crash victims, stands with others, during his funeral at a cemetery in St. Petersburg, Russia, Thursday, Nov. 5, 2015.  (AP Photo/Ivan Sekretarev)

There are increasing indications that the Russian airliner which crashed in Egypt's Sinai peninsula last week, killing all 224 people on board, was brought down by a bomb. According to an Agence France-Presse report, evidence from the plane's recovered black box suggests the flight came to a "violent, sudden" end after everything had been "absolutely normal" during its ill-fated journey.

If so, it raises troubling questions about security in the region and the threat of further terrorist attacks on Russia, which recently entered the Syrian conflict on behalf of the regime of President Bashar al-Assad.

Despite al-Qaeda's numerous failed attempts to bomb planes over the past decade, no militant group has successfully detonated a weapon inside a plane since a 2004 attack by Chechen suicide bombers in Russia. If the Russian Metrojet flight, which was bound for St. Petersburg, was downed by a bomb, it's a reminder of a worrying age of terrorism in the sky.

Here are some grim episodes:

Feb. 21 1970, Swissair Flight 330A bomb exploded in the cargo compartment of a flight from Zurich to Hong Kong, which had a scheduled layover in Tel Aviv. All 38 passengers and nine crew members perished; a Palestinian militant group claimed responsibility from Beirut.

Oct. 6, 1976, Cubana de Aviacion Flight 455A passenger jet from Barbados destined for Jamaica blew up soon after take-off, killing all 73 people aboard. A group of anti-Castro regime Cuban exiles -- potentially connected to the CIA -- were eventually convicted by Venezuelan authorities of carrying out the bombing.

Sept. 23, 1983, Gulf Air Flight 771A flight from the Pakistani city of Karachi to Abu Dhabi crashed near its destination after a bomb exploded in the baggage compartment. All 112 people on board died. Investigators ultimately linked the attack, which killed mostly Pakistani nationals, to a Palestinian militant organization that wanted to punish the Emiratis for not paying it protection money.

June 23, 1985, Air India Flight 182The large passenger jet, dubbed the "Emperor Kanishka" after an ancient South Asian king, took off from Toronto and was hit by a bomb above Irish air space and crashed in the Atlantic Ocean. All 307 passengers and 22 crew members died. The attack was carried out by Sikh separatist militants, supposedly as retribution for the hideous 1984 anti-Sikh riots in New Delhi.

April 2, 1986, TWA Flight 840: A long-haul flight from Los Angeles to Cairo was struck above Greece by a bomb planted beneath a seat. It blasted a hole in the plane's side, leading to the deaths of four Americans, including an infant, who fell out, but the bulk of the passengers survived as the pilot managed to make an emergency landing. The culprits were never properly identified, though a Palestinian group dubbed Arab Revolutionary Cells claimed responsibility, citing a struggle against "American imperialism."

Nov. 29, 1987, Korean Air Flight 858A flight between Baghdad and Seoul was brought down by North Korean operatives, who detonated a device using liquid explosives disguised in liquor bottles. All 115 people on board died.

Dec. 21, 1988, Pan Am Flight 103The transatlantic flight blew up above Scotland; its debris landed in the town of Lockerbie and killed 11 residents. All 259 people on board also died. The attack was linked to operatives from the regime of Libyan dictator Moammar Gaddafi, though investigations into the exact perpetrators are still ongoing.

Nov. 27, 1989, Avianca Flight 203A flight from the Colombian capital of Bogota to Cali blew up, killing everyone on board. The incident spotlighted the scope of the violence brought on by powerful, infamous drug lord Pablo Escobar; he was possibly hoping to kill a Colombian presidential candidate who ended up not taking the flight.

Aug. 24, 2004, Volga-Avia Express Flight 1353 and Siberia Airlines Flight 1047Two planes that flew out of Moscow were hit on the same day by a pair of female Chechen suicide bombers. A militant group dubbed the Islamist Brigades claimed responsibility for the attack, which killed 90 people in total.