The suitcase contained three laptops that had WikiLeaks materials, including evidence of a "war crime" allegedly committed by US troops in Afghanistan, according to an affidavit that Assange's lawyer filed along with a criminal complaint to police at Stockholm's Arlanda Airport.
In the affidavit, Assange suggested his bag may have been illegally seized "as part of an intelligence operation with the purpose of gathering information about me." He offered no proof but said all attempts to locate the bag had failed.
"The suspected seizure or theft occurred at a time of intense attempts by the US to stop WikiLeaks' publications of 2010," Assange said and suggested that Swedish authorities "seek explanations" from members of Obama's delegation during their visit.
The police border control division at Arlanda Airport opened an investigation as a matter of course after receiving the complaint today, spokeswoman Jessica Fremnell said.
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She declined to comment on Assange's suggestion to interrogate people in Obama's entourage, saying "we make our own decisions about what we think we need to do."
"It was checked in and we have luggage tags and everything," he said. "It's gone without a trace."
WikiLeaks spokesman Kristinn Hrafnsson said similar complaints would be filed in other countries, including Germany and Denmark, alleging potentially unlawful US intelligence operations against Assange.
"It's time to confront it and fight back on all fronts," Hrafnsson told the AP.
WikiLeaks has published tens of thousands of pages of secret US diplomatic and military communications over the last few years, deeply angering the Obama administration. US Army Pvt. Chelsea Manning, previously known as Bradley Manning, was recently sentenced to 35 years in prison for giving US government secrets to WikiLeaks.