"This we believe to be a natural death. Right now the evidence points to it," Nelson said.
He akso added that autopsy of Zyazin could take up to 8 weeks.
"If they can’t determine right off the bat what it was, then they would have to do toxicology and those types of reports can take up to six to eight weeks," Nelson said.
Investigators are not handling the death as a criminal investigation, Lt. Kenneth Nelson said.
"It is not a criminal investigation," Nelson said.
On Tuesday, ICE said in a statement that Zyazin, 46, died in custody on Sunday night at the Otay Mesa Detention Center. He had been detained by the US immigration agency since April 24.
According to the ICE, Zyazin had presented a Russian passport to US Customs and Border Protection officers at the port of entry to the country, after which it was determined that he had previously been deported in 2009.