MOSCOW – Families of the 89 people killed in two nearly simultaneous plane crashes in Russia were waiting yesterday for official word on whether their loved ones had been victims of a September 11-style terrorist attack or of a catastrophic coincidence.

Grief-stricken relatives and friends of the passengers converged on Moscow’s Domodedovo airport, where authorities were providing counseling and information behind closed doors. Both planes had departed from Domodedovo within an hour of each other late Tuesday bound for destinations in southern Russia. They went down at almost precisely the same moment about 480 miles apart. Relatives gathered at the airport doubted the twin crashes could have been an accident.

“I can’t believe that these two planes fell out of the sky at the same time by themselves. This has to be terrorism,” said Anatoly Zvagintsov, 53, whose 21-year-old daughter, Larissa, and her 27-year-old husband, Alexei, died when one of the planes went down. The couple had been heading to the Black Sea resort town of Sochi for a two-week honeymoon after getting married on Friday.

“The government has to do something about this, someone needs to bring order to this country,” Mr. Zvagintsov said.

[President Putin ordered security controls in Russia’s airports to be taken over by the Interior Ministry, according to a Bloomberg News report last night. Russia’s Federal Security Service said it still hadn’t ruled out terrorism as the cause of the synchronized plane crashes.]

As investigators sifted through the remains of the two planes, authorities said the possibility that the crashes resulted from terrorist attacks was being “carefully studied” along with other theories. One plane, a Sibir Airlines Tu-154 carrying 46 passengers and crew bound for Sochi, sent a hijack alert before crashing near the city of Rostov-on-Don, 600 miles south of Moscow. Witnesses on the ground reported hearing an explosion before the second plane, a Volga-Aviaexpress Tu-134 bound for Volgograd with 43 on board, went down near Tula, about 90 miles south of Moscow. Both planes disappeared from radar screens at about 11 p.m. Tuesday.

There were no survivors of either crash. All of the victims were Russian citizens except for an Israeli man, David Coen, who was on the Tu-134.

Security was tightened at all of Russia’s airports and at various public sites in Moscow following the crashes. Russian security forces are on high alert this week in advance of Sunday’s presidential elections in the breakaway southern republic of Chechnya, where Russian forces are waging war against separatist rebels. The rebels have claimed responsibility for a series of explosions and suicide bombings in recent years that has killed hundreds throughout Russia.

There were no immediate claims of responsibility for the crashes. Akhmed Zakayev, a London-based spokesman for Chechnya’s main rebel group, headed by former Chechen president Aslan Maskhadov, said yesterday that it was not responsible.

“To us any form of terrorism is absolutely unacceptable. We have condemned it and continue to condemn it,” Mr. Zakayev said. There was no word from extremist rebel leaders, including Shamil Basayev, a radical who has claimed responsibility for organizing past bombings and suicide attacks.

President Putin, who called short his vacation in Sochi to return to Moscow yesterday, ordered Russia’s Federal Security Service, or FSB, the former KGB, to investigate the crashes. Mr. Putin has declared today a national day of mourning.

“Before we begin our work, I would like to express my condolences to the families and loved ones of the victims,” Mr. Putin was quoted as saying by the ITAR-Tass news agency. Russia’s chief prosecutor, Vladimir Ustinov, told Mr. Putin yesterday that there was no clear indication of what caused the planes to crash.

“We are examining a number of versions, among them a terrorist act, and human and technical factors,” Mr. Ustinov said. “We don’t rule out any one of these theories today.”

The “black box” flight data recorders from both aircraft were recovered and are being analyzed.

The FSB’s spokesman, Sergei Ignatchenko, said that investigators had found no evidence of explosives during preliminary studies of the wreckage. Mr. Ignatchenko said the main line of inquiry involved “violation of the rules of operating civil aircraft,” including the use of poor quality fuel, breaches of fueling regulations, and pilot error. Investigators were questioning airport, airline, and security employees, as well as verifying the technical conditions of the planes and the content of the fuel used in both. Domodedovo airport said in a statement that both planes “went though the standard procedure of preparation for flight … [and] the procedures were carried out properly.”

Sibir and Volga-Aviaexpress also rejected the possibility of human error or technical faults, saying that the planes were properly checked before the flight and had experienced crews. Southern Russia had been hit with thunderstorms at the time of the crashes and investigators were also looking at bad weather conditions as a possible cause.

Rainy conditions hampered search and recovery operations, in which more than 1,500 servicemen were involved. By late yesterday, the bodies of all 43 killed in the Tula crash had been recovered, but the recovery effort in Rostov was taking longer as the wreckage was spread over several miles. The American homeland security secretary, Tom Ridge, discussed the crashes at a visit to police headquarters in New York yesterday.

“Obviously it’s far too early to determine causation.We’ve also volunteered if there is forensic capability that we have in any of our agencies – the National Transportation Safety Board can be of any assistance to them – obviously any resources we have that can help them determine causation. We made the offer; it’s up to them to determine whether they accept it.”