Michael Lauber told reporters he did not rule out searching the headquarters of UEFA, the governing body of European soccer, as part of the investigation.
Swiss prosecutors said on Friday they had opened a criminal investigation into Blatter, the long-time head of world soccer's governing body FIFA, on suspicion of criminal mismanagement and misappropriation of funds.
The prosecutors said Blatter was suspected of a "disloyal payment" of 2 million Swiss francs ($2.05 million) to Platini in 2011 at the expense of FIFA, allegedly made for work performed between January 1999 and June 2002.
Platini, who until Friday was considered favourite to win the presidential election on Feb. 26, also asked to give information.
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"We didn't audition Mr Platini as a witness, that's not true. We investigated against him in between as a witness and an accused person," Lauber said after a speaking engagement on Tuesday.
"If I was satisfied or not, I can't tell because I would do real damage to the investigation."
He said his office had not yet decided whether to bring charges against Blatter.
"I will do anything if I can do something to clear up ...the real truth," he said when he was asked if UEFA's lakeside headquarters in Nyon could be searched. "If I have enough elements to go there, I could not exclude that."
UEFA said on Friday Platini had been interviewed as a witness.
On Monday, Platini said in a letter to UEFA's 54 member associations he had been interviewed "not as a person accused of any wrongdoing but simply in my capacity as a person providing information".
(Writing by Brian Homewood; editing by Andrew Roche)
