The Department of Justice probe into the 66-year-old diplomat is thought to be the first into a serving British ambassador in Washington.
It was launched after top US government officials were “astonished” by a series of articles by Kosinski, 46,
One story about plans for a top–secret visit to North Korea by Secretary of State Mike Pompeo sparked horror, with US security interests said to be at risk.
Lord Darroch, married for 42 years and with two grown-up children, is believed to have fiercely denied any impropriety and any suggestion he was the leaker — and he was eventually cleared.
Ironically he quit his post in July last year after his own diplomatic cables were leaked, showing he had branded President Donald Trump “inept” and “incompetent”.
Kosinski’s public LinkedIn profile shows she left her post at CNN five months later.
A security source said: “The leaks of the cables last year were obviously deeply embarrassing and left Darroch no option but to resign once President Trump made his feelings clear.
“But for the US to investigate the most senior diplomat of its greatest ally on suspicion of leaking secret information is almost unprecedented.
“Although the investigation found nothing over Darroch, the fact it happened is extraordinary.”