One presenter, Lenin Artieda, was injured when he opened the envelope in the middle of the newsroom.
He said the explosive device looked like a USB drive. He plugged it into his computer and it detonated.
The Ecuadorean attorney-general’s department confirmed it had opened a terrorism investigation into the letters on Monday.
It did not name the specific news outlets targeted. However, at least five different organisations across Ecuador were sent the letters.
The government has condemned the attacks, describing freedom of expression as “a right that must be respected”.
The interior minister, Juan Zapata, said the devices were all sent from the same town. Three were sent to media outlets in Guayaquil and two to the capital, Quito.
While Mr Artieda was injured by the device, others sent through the post failed to explode or were never opened.
Police carried out a controlled detonation of one of the devices sent to TC Television, prosecutors confirmed.
Ecuador’s head of forensic science said they contained “military-type” explosives.
Ecuador has experienced an increase in violence, which its President, Guillermo Lasso, said is a result of competition between drug trafficking gangs for territory and control.
Guayaquil, Ecuador’s second-largest city, where three of the explosives were sent, has experienced dramatic levels of violence including decapitated bodies hanging from pedestrian bridges and deadly prison riots between rival gangs.
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