BUCHALKI, Russia - Russian emergency workers searched heaps of twisted metal and tall grass today for clues to what caused two airliners to plunge to Earth almost simultaneously, killing all 89 people aboard. Officials said one jet sent a hijack distress signal, raising fears terrorists had struck.

Flight recorders from both planes were found and taken to Moscow for investigation, ITAR-Tass reported, indicating the question of what caused the twin disasters soon could be answered.

Russia's main intelligence agency, however, said it had found no evidence of terrorism in initial investigations at the crash sites. The Federal Security Service, or FSB, said it was investigating other possibilities such as technical failures, the use of poor quality fuel, breaches of fueling regulations and pilot error, its press service told The Associated Press. Rain and thunder was reported in the regions where both crashes occurred.

Rebels fighting a protracted war for independence for Chechnya, the troubled southern Russian province, have been blamed for a series of terror strikes that have claimed hundreds of lives in Russia in recent years. But rebel representative Akhmed Zakayev told Russia's Ekho Moskvy radio from London that Chechen forces and rebel leader Aslan Maskhadov were not connected to the crashes.

Russian officials had expressed concern that separatists in the war-ravaged republic might carry out attacks ahead of a regional election Sunday to replace its pro-Moscow president who was killed in a May bombing.

A Sibir airlines Tu-154 jet, carrying 46 people, took off from Moscow's newly redeveloped Domodedovo airport at 9:35 p.m. Tuesday and the other plane, a

Volga-Aviaexpress

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 is vacationing.

The planes disappeared from radar screens about 11 p.m., and by early this 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."

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

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. The FSB sent experts to determine if explosions caused the crashes, Interfax reported.

At about the same time the Tu-154 disappeared, the Tu-134 airliner crashed in the Tula region, about 125 miles south of Moscow, officials said. ITAR-Tass reported that the authorities believe the Tu-134 fell from an altitude of 32,800 feet. Wreckage of the Sochi-bound Tu-154 was found in the Rostov region, about 600 miles south of Moscow.

Officials made conflicting statements about whether the signal from the other jet indicated a hijacking or another severe problem on the aircraft.

The Interfax and ITAR-Tass news agencies later quoted an unnamed law enforcement source as saying that the signal was an SOS and that no other signals were sent.

Sibir airlines, however, seemed to hint at foul play, saying on its Web site that it "does not rule out the theory of a terrorist attack."