A source close to the investigation into President Donald Trump’s campaign and its ties to Russia says that there is now “specific concrete and corroborative evidence” that individuals within Trump’s immediate orbit coordinated with Russian intelligence operatives during the election.

The Guardian said on Thursday that the U.K.’s Government Communications Headquarters (GCHQ) has been watching the Trump camp since late 2015, having noticed suspicious “interactions” between Trump associates and well-known Russian agents.

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GCHQ passed the information it had uncovered along to U.S. intelligence agencies as part of the routine exchange of information and for the next six months until summer of 2016, U.S intelligence agencies were receiving information about the contacts from European agencies in Germany, Estonia and Poland.

More information came in to U.S. hands from members of the “Five Eyes” alliance that includes the U.S., the U.K., Canada, Australia and New Zealand.

“It is understood that GCHQ was at no point carrying out a targeted operation against Trump or his team or proactively seeking information,” the Guardian said. “The alleged conversations were picked up by chance as part of routine surveillance of Russian intelligence assets. Over several months, different agencies targeting the same people began to see a pattern of connections that were flagged to intelligence officials in the US.”

However, after carefully compiling the assembled intelligence, one source close to the investigation said, “They now have specific concrete and corroborative evidence of collusion. This is between people in the Trump campaign and agents of [Russian] influence relating to the use of hacked material.”

Sources close to the GCHQ told the Guardian that it looks like U.S. intelligence agencies “were asleep” with regards to Russia and its alliance with the Trump team.

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“They [the European agencies] were saying: ‘There are contacts going on between people close to Mr Trump and people we believe are Russian intelligence agents. You should be wary of this,’” the source said. “The message was: ‘Watch out. There’s something not right here.’”

U.S. spy agencies, however, are forbidden from spying on citizens without a warrant — warrants that are very difficult to obtain.

“They are trained not to do this,” the source said, which slowed down the time it took for U.S. agencies to become aware of the threat and respond, which it did in the late summer of 2016.