Russian investigators said they found no evidence Wednesday that terrorists brought down two passenger jets that crashed almost simultaneously Tuesday night, killing all 90 people aboard, and suggested that the planes' loss could be an improbable coincidence of technical malfunction or human carelessness.

Investigators searching the grassy fields where the airplanes fell, nearly 500 miles apart from one another, recovered flight recorders but discovered no signs of sabotage, officials said. Although not ruling out terrorism, authorities opened a criminal investigation into possible negligence with the transportation minister in charge of the probe.

Despite the government's statements, many Russians, including some aviation experts, considered terrorism the only plausible explanation. The crashes, which came five days before a regional election in the separatist region of Chechnya, fueled suspicions that this was the latest in a two-year wave of Chechen terror assaults that have claimed more than 500 lives.

Russian security forces initially responded as if terrorists were responsible, stepping up security at checkpoints in Chechnya and at airports around the rest of Russia.

Sibir Airlines, which operated one of the lost planes, said it was told by the government that the craft sent a signal just before the crash indicating that it had been hijacked, but Russian officials said they could not confirm that and added that it could have been a general distress signal.

Investigators said they were checking into the condition of the aircraft, the type of fuel used, weather conditions and pilot performance, as well as potential sabotage. Russia's aging fleet of civilian airliners has been a source of concern for years.

"I just think we're talking about negligence," Sergei Ignatchenko, chief spokesman for the Federal Security Service, the domestic successor to the KGB, said in a telephone interview. "Our planes have already used up their resources. Unfortunately, they're still used and they're still flying."

But Ignatchenko also acknowledged the improbability of catastrophe affecting two planes at the same time without terrorist intervention. "It's too much of a coincidence," he said. "We're not denying terrorism. It's one version and we're checking it, of course. But as of now on the sites, we haven't discovered any explosives or any trace of any violence. That's why we're saying the main reason is the violation of safety rules."

President Vladimir Putin, who was heavily criticized for remaining on a vacation when the nuclear submarine Kursk sank in 2000, broke off his holiday at the Black Sea resort town of Sochi to return to Moscow. But in televised remarks, he, too, seemed to discount terrorism, ordering aides to give him "objective, reliable information" and assist the families of the victims.

The tone contrasted starkly with Putin's standard response to apparent terrorist attacks, which he often blames on Chechen rebels before an investigation has begun. When a bomb killed more than 40 people on the Moscow subway in February, Putin said that "no circumstantial evidence is needed" to accuse Chechen terrorists, and he vowed to "liquidate them."

The Chechen separatist leader, Aslan Maskhadov, promised this summer to escalate attacks against Russian targets, but on Wednesday his representative denied any involvement with the plane crashes. "There is no way there could be any connection," Akhmed Zakayev said by telephone from London. "When a terrorist attack happens, even if there are no direct connections, they will always find a Chechen trail."

Analysts noted that Maskhadov does not control all Chechen guerrillas, many of whom follow the guerrilla commander Shamil Basayev.

Either way, Alexei Malashenko, a scholar at the Carnegie Moscow Center, said "of course" the crashes were connected with the Chechen elections. "They deeply want to show everybody they are very strong, they're organized."

Chechen rebels have hijacked planes before, but never destroyed one with a bomb or used one in a suicide attack. If that were shown to be the case this time, "it means it is a kind of evolution of terrorism in Russia," Malashenko said. "They use the experience from abroad, the experience of al Qaeda and the Middle East."

Aircraft specialists were skeptical that two planes could have failed on their own at the same time. "Theoretically it's possible, but only theoretically," said Yuri Chervakov, an engineer at the company that makes MiG warplanes. "The chances are so tiny that we can't talk about it seriously."

The two planes took off from Moscow's Domodedovo Airport within about 40 minutes of each other Tuesday night, bound for separate destinations in southern Russia. Volga-Aviaexpress Flight 1303, a Tu-134 heading for Volgograd with 35 passengers and nine crew members, disappeared from radar screens at 10:56 p.m., according to Irina Andrianova, a spokeswoman at the Ministry of Emergency Situations. Sibir Flight 1047, a Tu-154 heading for Sochi with 38 passengers and eight crew members, vanished three minutes later.

The first plane smashed into the ground about a mile from the village of Buchalki in the Tula region, more than 100 miles south of Moscow. Residents described hearing a rumbling series of explosions similar to thunder, a noise officials said was probably the sound of the crash rather than a bomb blast.

"There were sort of knocks on the window, boom-boom," Nikolai Gorokhov, a resident, told NTV television. "I thought it was a thunderstorm again, and then there was a very long drawn-out roar and then everything died out."

The wreckage was scattered around a field of tall grass, the tail about half a mile from the plane's body. A book with its pages partially torn fell on one man's garage. Two bodies landed in a woman's field.

The second plane came down in a remote area near the village of Gluboky about 82 miles north of Rostov-on-Don. The spot was so isolated that rescue squads were not able to find it until well past dawn, nine hours after the crash. A tattered shoe lay among giant shards of ripped metal.

Bewildered relatives of passengers and crew spent the day trying to find out what had happened to them. The gas monopoly Gazprom lost its local general director in Volgograd and another executive. "For our company, for our team, it's a real shock," said Sergei Ledashov, an official with the firm. "This was an absolutely regular business trip. They made it all the time."

At Domodedovo Airport, a handful of relatives were shepherded Wednesday to a makeshift information center in a decrepit building. They pored over lists amid a huddle of police, airport officials, psychologists and journalists. One man, who declined to give his name, said he found a missing friend's name on a reservations list, but Sibir could not confirm that he had boarded the plane because he was not on a still-incomplete passenger list.

At midafternoon, airline officials fielded frantic phone calls and continued trying to figure out exactly who had been on board. "As you would expect, people are in a panic," said Maxim Yeryomenko, a security official at the information center.

Regardless of the cause, some politicians said the episode demonstrated Russia's vulnerability to terrorism and should compel the government to do more to protect its citizens. "President Putin has done what he could," said Gennady Gudkov, a former KGB officer who now serves in parliament. "But today we have to make more efforts. They need to be bigger."

Correspondent Peter Finn contributed to this report.

Russian soldiers pass debris from a Tu-134 airliner that crashed Tuesday night about a mile from the village of Buchalki in the Tula region, south of Moscow.