Maria Paula Romo did not identify the person but told Sonorama radio he was very close to Assange, who was arrested Thursday in London on a US extradition request after spending seven years in the Ecuadoran embassy there.
Channel Teleamazonas identified him as Ola Bini, a software developer focused on privacy, security and cryptography, but did not name its source.
He "has been detained simply for investigation purposes," she said, adding he had taken foreign trips with former Ecuadoran foreign minister Ricardo Patino, who gave political asylum to Assange in 2012.
"We have sufficient evidence that he was collaborating in attempts to destabilize the government," she said.
Romo's announcement followed the arrest of Assange earlier Thursday.
He had been living in the embassy, fearing arrest, but the Ecuadoran government says that Assange "repeatedly violated" the conditions of his protection.
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