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The accident that killed 16 people and injured 21 others over the weekend on the Mexico City-Tampico highway was caused by speeding and poor maintenance of the vehicle, state officials said Monday.

The accident occurred Sunday near Corredor de la Montaña, a town in Hidalgo state.

The vehicle's tires were bald and it was in "bad condition," an Hidalgo government spokesman told Efe.

Two of the injured are in serious condition, the official said, adding that eight others would be released later in the day from the hospitals where they were treated in Pachuca, the capital of Hidalgo.

The bus was returning to the capital from the resort town of Santa Maria Amajac and all of the passengers were from the Mexico City borough of Iztacalco, the state spokesman said.

Eleven people were killed and nine others injured last Thursday when a bus plunged into a ravine near Temuxco, a town in Hidalgo. EFE