That's because, given what we know so far about Russia's efforts to interfere in the election, any coordination with the Trump campaign could have been accomplished in just a few face-to-face meetings.
Any digital communications between the two camps, Levin said, would be hard to sweep up, given how meticulously foreign intelligence services tend to hide their tracks when interfering in other countries' democratic processes.
Levin cited American intervention in the Chilean presidential election in 1964. The U.S. spent millions of dollars in a series of covert missions designed to defeat democratic socialist Salvador Allende in the country's contest that year.
The American efforts were not proven until the 1975 Church Committee report from the U.S. Senate — and Eduardo Frei, the country's CIA-backed candidate who won the 1964 election, continued to deny receiving any assistance until his death in the 1980s. The White House and the CIA declassified documents related to the election meddling efforts in 2004.
Allende won the 1970 Chilean presidential election. In 1973 he was overthrown in a military coup.
"Mueller would have to find evidence of a handful of private meetings with few participants, which were conducted with people who have advanced professional training in hiding their tracks and avoiding surveillance," Levin wrote. "Likewise, given the possible non-digital nature of such contacts, Mueller's team would have to do this without a critical investigatory tool: advanced U.S. signals intelligence and cyber-warfare capabilities."
Levin notes that, historically, election meddling does involve some collusion, with some exceptions.
"In the vast majority of post-World War II electoral intervention cases for which there is sufficient evidence to examine this aspect at some reasonable level of certainty, I found such collusion to be occurring," he wrote.
But, he added, if any occurred, conclusive evidence could take decades to surface.
The special counsel's office declined to comment.
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