(Above) Russian women weep at news of the casualties list in Moscow.

(Above) Russian transportation official removes the flight recorder.

(Above) Workers sift through the debris. Note the severely worn tires. Could be fire damage from after the crash, I don't know.

(Above) A Russian Tupolov-134 flight recorder. Interesting to me that it has English stenciled on the container.

MOSCOW, Russia (CNN) -- Explosives have been found in the wreckage of the second of two jets which crashed almost simultaneously this week, Russia's FSB security service told CNN.

"Additional examination of the fragments of the Tu-134 aircraft which crashed on Tuesday ... has revealed traces of hexogen," an FSB spokesman said.

The FSB said on Friday that hexogen, more widely known as RDX, had been found in the wreckage of the other Tu-154 plane which crashed on Tuesday in southern Russia.

CNN's Paula Hancocks said that the investigation was now firmly focused on terrorism as the likely cause of the crashes.

She said hexogen has been used in previous attacks in Russia blamed on Chechen separatists -- including the 1999 apartment bombings in Moscow.

Until now. Russian Investigators have carefully avoided any suggestion that Chechen militants were behind the crashes.

But Russian media have speculated that two passengers, believed to be Chechen women, blew up the planes in the run-up to Sunday's Chechen election certain to return a pro-Moscow president.

Moscow toughened security measures for airline flights Saturday and vowed to prevent any recurrence of the twin air crashes.

Transport Minister Igor Levitin said his concern was to ensure safe air travel. Safety measures, previously undertaken solely by airports, would now be shared with the Interior Ministry.

"From today, they (Interior Ministry officials) are being included in teams conducting searches," Levitin, ordered by President Vladimir Putin to head a commission investigating the crashes, said in an interview.

"We want to toughen all requirements in terms of cargo and baggage ... Passengers must be made to feel that everything is in order once they are seated in an aircraft."

Authorities understood that the virtually simultaneous crashes were "an extraordinary event ... We must look thoroughly into this to understand what happened and take measures to ensure it does not happen again."

Levitin said ministers wanted to enshrine tough regulations in a law on air safety. Airports were to be provided with new detection equipment partly financed by the government.

The FSB had earlier said traces of an explosive used in past attacks blamed on Chechen militants were found in the wreckage of the Tu-154 plane, which crashed en route from Moscow to the Black Sea resort of Sochi.

Later it said the same explosive had been found at the crash site of the Tu-134 plane which came down on its way to Volgograd in central Russia.

NTV television, reporting from the Tu-134 crash site, earlier said investigators were leaning towards the notion that the second aircraft had also been blown up.

The television showed coffins draped with wreaths in Volgograd, with groups of women dressed in black seated nearby. At least one funeral was shown taking place in Sochi.

Russian media said investigators were trying to determine whether two women with Chechen names were linked to the crashes. The daily Izvestia reported that the brother of one woman had been seized by Russian forces in Chechnya three years ago.

Chechen separatists have been blamed for numerous bombings and other attacks in Russia in recent years, including the seizure of hundreds of hostages at a Moscow theater that ended with more than 100 hostages dead.

Chechen Interior Minister Alu Alkhanov, backed by the Kremlin, is almost sure to win Sunday's poll, called to replace a president assassinated in May. He faces six obscure rivals.