The militants used Google to check Mr Martinkus's claim that he was working for Australian public broadcaster SBS.
"They had thought he was working for the Americans as an informer," said Mike Carey, executive producer at SBS.
"In this case, modern technology probably saved the journalist's life."
Mr Martinkus, who had been captured at gunpoint outside the al-Hamra hotel in Baghdad, was eventually released on Sunday.
More news from dotJournalism:
Online publisher dares US government contractor
Another Iraq combat video surfaces
Baghdad training project forced to close
Plea to help solve Iraqi 'torture' riddle
See also:
Silicon.com coverage: http://networks.silicon.com/webwatch/0,39024667,39125122,00.htm
SBS: http://www.sbs.com.au
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