Русско-англо-русский юридический словарь

Not so fast, say the Russians, who act in collusion with the European Union to seize Norway’s oil and natural gas installations, inaugurating a “soft occupation” of the country. (The Washington Post)

Collusion is not a crime, but colluding can be. In the colloquial sense, collusion simply means an act of secret cooperation, generally for nefarious ends. There’s no law you can violate that constitutes collusion in the third degree — but working with someone in a conspiracy to break the law is illegal.

It’s called conspiracy.

“Every conspiracy is a form of collusion,” Stanford law professor Robert Weisberg said when we spoke by phone on Tuesday. “Conspiracy is an agreement to do a crime plus some overt act in that direction. It’s really that simple.” (The Washington Post)

I don't have sufficient information to make a determination whether or not such cooperation or complicity or collusion was taking place. (CNN Transcripts, Live Coverage of Former CIA Director John Brennan's Testimony Before Congress)