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The report quoted Middle East sources who said the plane, which took off from Tehran and was headed for Armenia, was carrying in its luggage compartment several metal cases with two kilograms of explosives and detonators destined for the Lebanese terrorist group.
http://www.israelnationalnews.com/News/News.aspx/132682

Can this be true? Is it really likely that they had explosives and detonators in the same cases?

This is probably the reason:
The Tupolev’s three engines are in its tail section. The flames there could indicate “an uncontained engine failure,” said Patrick Smith, a pilot and the air travel and safety writer for Salon.com.

The average age of Iran's fleet of aircraft is 22 years, said Masoud Mohajer, an aviation expert in Tehran. Age itself may not be a problem - even older jets are in service around the world - but keeping them maintained is. Mohajer said Iranian airlines can't afford to keep even Russian planes in shape because of lack of government support.

He pointed to “the financial inability to buy new planes or carry out maintenance requirements.”

“Iranian airliners don't have enough cash even to buy new Russian planes. The government controls ticket prices. It's not profitable for airliners,” Mohajer said.
http://news.yahoo.com/s/ap/20090715/ap_on_re_mi_ea/ml_iran_plane