BUCHALKI, Russia — Russian investigators labored Wednesday to determine whether terrorism caused the near-simultaneous crashes of two jetliners, killing all 89 people aboard and spreading anxieties about a possible bloody escalation of the Chechen conflict.

Officials stressed that no evidence of a terrorist attack had yet been found among charred wreckage and said that they opened a criminal investigation as they looked into other causes like bad fuel, equipment malfunction and human error. The planes’ data recorders were recovered, but experts were only starting to retrieve information from them.

The planes crashed just days before a Kremlin-called presidential election in Chechnya, whose rebels have staged suicide bombings and other attacks across Russia in recent years, including the 2002 seizure of hundreds of hostages at a Moscow theater.

Witnesses reported hearing three explosions before a Volga-Avia express airline Tu-134 went down in a field near Buchalki, about 125 miles south of Moscow, with 43 passengers and crew.

The wreckage of a Sibir airlines Tu-154 with 46 people aboard was spread over a few hundred yards in a rugged field near Gluboky in the region of Rostov-on-Don, some 600 miles south of Moscow. The Tu-154 jet had activated a signal indicating the plane might have been hijacked or in distress.

Reports of far-flung wreckage suggest an explosion may have preceded a crash, said Jim Burin of the U.S.-based Flight Safety Foundation. He also said bad fuel could cause an airplane’s engines to fail, but the crew likely would have reported it well before engines quit.

“I would expect some communication from the crew that we’re having trouble,” he said.

Russian authorities had expressed concern that Chechen separatists might stage new attacks before the Sunday vote, but there was no rush by officials to tie the crashes to Chechnya — a determination that would underline the government’s failure to quell the decade-old insurgency.

“Several versions are being examined, including a terrorist attack, and other possibilities — the human and technical factor,” Russia’s top prosecutor, Vladimir Ustinov, told President Vladimir Putin during a televised meeting about the Tuesday night crashes.

Ustinov said that the Prosecutor General’s office had instituted criminal proceedings into both crashes and sent two teams of investigators to the crash sites, the ITAR-Tass news agency reported. The teams are headed by his deputies.

“We have not rejected any of the leads,” Ustinov said, according to the news agency.

Putin, who expressed sympathy for the families of the dead, did not publicly address the terror question.

After designating Thursday a national day of mourning, he ordered the Federal Security Service to investigate the crashes and said he wanted “unbiased and reliable information” from the investigation.

The service is a successor agency to the KGB.

While officials spoke cautiously on the terrorism issue, Russian police said security was being tightened at airports and other transport hubs and public places.

The planes took off about 40 minutes apart from the single terminal at Moscow’s newly renovated Domodedovo airport, which is about 14 miles outside of Moscow They both crashed a few minutes apart just before 11 p.m., according to initial time estimates.

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.”

In Washington, U.S. officials said they had no information on the disaster, but said American agencies were ready to provide help if asked.

“Our understanding is, there is no cause that has been ruled in or no cause that has been ruled out,” State Department deputy spokesman Adam Ereli said.

Outside experts expressed skepticism that anything but violence could be behind two planes crashing at almost the same time hundreds of miles apart.

“That’s pretty far out there on the chance bar,” said Bob Francis, former vice chairman of the U.S. National Transportation Safety Board.

Rafi Ron, former head of security at Israel’s Ben Gurion Airport and now a security consultant in Washington, said he was convinced it was terrorism.

“The timing indicates that this is probably a coordinated attack,” he said.

He also noted reports that one of the jetliners activated an emergency signal shortly before disappearing from radar screens, which could indicate a hijacking.

“In my assumption, that must have been the result of a terrorist on board,” Ron said.

Oleg Yermolov, deputy director of Russia’s Interstate Aviation Committee, said it was impossible to judge what was behind the signal, which he said is used merely to indicate “a dangerous situation onboard,” including possibly a catastrophic mechanical problem.

Aviation officials also said the crew of the other plane gave no indication anything was wrong, although people on the ground reported hearing a series of explosions.

Appearing on television, FSB spokesman Sergei Ignatchenko said investigators picking through the wreckage scattered in tall grass had so far not found any evidence of a terrorist attack.

——— Associated Press reporter Sergei Venyavsky in Gluboky contributed to this article.