The operator of one of the two planes that crashed almost simultaneously in Russia said Wednesday that early indications were that its aircraft had exploded in midair.

"The wide distribution of large fragments indirectly confirms the conjecture that the plane broke up in midair because of an explosion," Sibir Airlines (search) said in a statement of its Tu-154 jet, Reuters reported.

The statement came as Russian emergency workers continued to search through heaps of twisted metal and tall grass Wednesday for any hints as to what caused two airplanes to crash, killing all 89 people aboard.

Officials said one of the jets sent a distress signal that may have indicated a hijacking.

Rescuers found the four flight recorders from the two planes late on Tuesday and brought them to Moscow for analysis. Those flight recorders could give investigators more insight into what happened aboard the planes before they plunged.

Russian security authorities said that explosives specialists were still working at the scene of the crashes. They reported that terrorism remained a possible cause, although there was no evidence so far that terrorists were behind the tragedies.

Russia's main intelligence agency, the Federal Security Service (search), said it had found no evidence of terrorism in initial investigations at the crash sites. The FSB — formerly known as the KGB — said it was investigating all possible causes, such as technical failures, the use of poor quality fuel, breaches of fueling regulations and pilot error.

Federal Security Service spokesman Sergei Ignatchenko said investigators were still questioning airport officials and airline and security employees at Domodedovo Airport, from which both flights left 45 minutes apart.

The airport on Moscow's far south side operates a single terminal that serves both international and domestic flights. Both flights were serviced at and left from the domestic section. Rain and thunder were reported in the regions where both crashes occurred.

But the Interfax news agency quoted an unnamed Russian aviation security expert as saying the fact that the two planes disappeared around the same time raised suspicions of terrorism.

A former member of the U.S. National Transportation Safety Board (NTSB) told FOX News that the coordinated crashes and the fact that the jets took off from the same airport raises suspicions.

"That sounds a lot like 9/11," Vernon Grose told FOX News on Wednesday. "We see some parallels there."

A Sibir airlines Tu-154 jet, carrying 46 people, took off from Moscow's (search) newly redeveloped Domodedovo airport (search) at 9:35 p.m. Tuesday and the other plane, a Tu-134 carrying 43 people, left 40 minutes later, according to state-run Rossiya television. The Tu-134 was headed to the southern city of Volgograd, while the other plane was flying to the Black Sea resort city of Sochi, where President Vladimir Putin was vacationing.

Putin returned to Moscow Wednesday evening and met with chief prosecutor Vladimir Ustinov, who told him authorities were considering terrorism, technical problems or human error as possible causes of the crashes. Putin announced that Thursday would be a day of mourning, the Interfax news agency reported.

The planes disappeared from radar screens about 11 p.m., and by early Wednesday morning, the wreckage of both had been found — with no survivors. Domodedovo airport said in a statement that both planes "went through the standard procedure of preparation for flight ...(and) the procedures were carried out properly."

Signal Noted 'Dangerous Situation Onboard'

Sibir airlines said on its Web site that it "does not rule out the theory of a terrorist attack." The airline is one of Russia's largest.

Uncertainty over the cause of the crashes came after Sibir — which means Siberia — said that it was notified that its jet had activated a hijack or seizure signal shortly before disappearing from radar screens.

Oleg Yermolov, deputy director of the Interstate Aviation Committee, said that it is impossible to judge what is behind the signal, which merely indicates "a dangerous situation onboard" and can be triggered by the crew during a hijacking or a potentially catastrophic technical problem.

Officials said the crew of the other plane gave no indication that anything was wrong, but witnesses on the ground reported hearing a series of explosions.

"There were three loud bangs on the window, like someone knocking," said Nikolai Gorokhov, a local resident who was in his home at the time of the crash.

Putin ordered an investigation by the FSB, and security was tightened at Russian airports, where extra security officers and sniffer dogs were called in to check passengers and luggage, as well as other transport hubs and public places.

Rescuers quickly found the Tu-134's wreckage — a heap of metal lying upside down in a large hay field, its tail severed from the fuselage. An AP reporter saw one body bag lying near the tail, holding a charred corpse. Emergency Ministry officers wearing camouflage and red berets stood shoulder-to-shoulder and combed the tall grass for pieces of the broken plane.

The Emergency Situation Ministry's Rostov regional chief Viktor Shkareda told AP the plane apparently broke up in the air and that wreckage was spread over an area of some 25-30 miles, but the fuselage and tail lay a few hundred yards apart at the edge of a forest. Bodies lay near the plane, but most of the victims' bodies were trapped in the mangled fuselage. The crash was found near Gluboky, a village north of the regional capital Rostov-on-Don.

Interfax quoted a Domodedovo airport spokesman as saying no foreigners were on the passenger lists for either plane. But a spokesman for the Israeli embassy said an Israeli citizen, David Coen, was on the Volgograd-bound jet.

Chechen Elections Spark Terrorism Speculation

Many people on the street believe the crashes were the results of terrorist acts, given that they occurred just a few days before a major election.

Officials had expressed concern that separatists in war-ravaged Chechnya might carry out attacks ahead of a regional election Sunday to replace the pro-Moscow president who was killed in a May bombing. Chechen rebels have been blamed for a series of terror strikes that have claimed hundreds of lives in Russia in recent years.

Rebel representative Akhmed Zakayev told Russia's Ekho Moskvy radio from London that Chechen rebel forces and rebel leader Aslan Maskhadov (search) were in no way connected to the near simultaneous crashes.

FOX News' Steve Harrigan and The Associated Press contributed to this report.