Veterinarians suspect the animals were suffocated by exhaust fumes in the enclosed truck, which was heated to counter outside temperatures below minus-30 degrees Fahrenheit (Celsius).
Circus officials and police initially said that eight Bengal tigers had died.
But Mechta circus administrator Yevgeny Kudashkin said there were seven tigers and two were Siberian tigers, which some conservationists fear may be approaching extinction.
The loss of the endangered tigers -- also known as Ussuri, Amur or Manchurian tigers -- was certain to increase the outrage over the animals' deaths.
A lioness also died during the trip across Siberia to the city of Yakutsk, where the circus was due to perform.
The animals appear to have been killed by exhaust fumes, Darya Kokhunova, deputy director of the Irkutsk veterinary laboratory, said. She cautioned that the tests were not yet complete. The Siberian tiger is rapidly disappearing from the forests of Russia's Far East and the Chinese province of Manchuria due to poaching and loss of habitat.
The New York-based Wildlife Conversation Society estimates that only 300 remain in the wild. They are the largest tiger species, weighing up to 600 pounds (270 kilograms).
Russian Prime Minister Vladimir Putin has done much to draw attention to their plight. During a visit to a wildlife preserve in 2008, he shot a female tiger with a tranquilizer gun and helped place a transmitter around her neck as part of a program to track the rare cats.
Later in the year, Putin was given a 2-month-old female Siberian tiger for his birthday. State television showed him at his home gently petting the cub, which was curled up in a wicker basket with a tiger-print cushion. The tiger, called Mashenka, now lives in a zoo in southern Russian.
Tigers remain a popular attraction in many of the dozens of permanent and traveling circuses in Russia.