"The indications and analysis so far of the sound on the black box indicate it was a bomb. We are 90% sure it was a bomb," said the Egyptian investigation team member, who asked not to be named due to sensitivities, Reuters wrote.
Asked to explain the missing 10 %, the investigator said: "I can't discuss this now."
Islamic State militants fighting security forces in Egypt's Sinai Peninsula have said they brought down the Airbus (AIR.PA) A321, which crashed 23 minutes after taking off from the resort of Sharm al-Sheikh a week ago en route to St Petersburg, killing all 224 passengers.
Egyptian officials say they are examining all possible scenarios on what could have caused the disaster but have cautioned against jumping to conclusions.