Justin Trudeau and Gerald Butts have been friends dating back to Butts's student days at McGill University in Montreal.
But today, they've parted ways as Butts, 47, tendered his resignation as principal secretary to Trudeau.
"Recently, anonymous sources have alleged that I pressured the former Attorney General, The Honourable Jody Wilson-Raybould, to assist SNC-Lavalin with being considered for a deferred prosecution agreement," Butts wrote. "I category deny the accusation that I or anyone else in his office pressured Ms. Wilson-Raybould.
"We honoured the unique role of the Attorney General," he continued. "At all times, I and those around me acted with integrity and a singular focus on the best interests of all Canadians."
Butts went on to say that he served the prime minister to the best of his abilities and gave him "free and unfettered advice".
"I have served the public interest, not the interests of any individual or any narrow private interest of any kind, at any time," Butts insisted. "Life is full of uncertainties, but I am absolutely certain of that."
Why did he resign? He stated that the prime minister's office "is much larger and more important than any of its staff".
"Any accusation that I or the staff put pressure on the Attorney General is simply not true," Butts reiterated. "Canadians are rightly proud of their public institutions.
"They should be, because they work," he continued. "But the fact is that this accusation exists. It cannot and should not take one moment away from the vital work the Prime minister and his office is doing for all Canadians.
"My reputation is my responsibility and that is for me to defend. It is in the best interests of the office and its important work for me to step away."
He pointed out that he encouraged Wilson-Raybould to run as a candidate for the Liberals. He also claimed to have supported her in cabinet.
"From my perspective, our relationship has always been defined by mutual respect, candour, and an honest desire to work together."
Near the end of his letter, Butts discussed the importance of addressing climate change.
"Our kids and grandkids will judge us on this one issue above all others," he wrote.
It was an unusual comment, given that the Trudeau government bought the Trans Mountain pipeline system from Texas-based Kinder Morgan for $4.5 billion.
The Liberal government is also dedicated to completing a $9.3-billion Trans Mountain expansion project that will triple bitumen shipments from Alberta to B.C. for export abroad.
In addition, the Liberal government approved the carbon-spewing LNG Canada project near Kitimat and retained the same greenhouse gas reduction targets as the former Conservative government led by Stephen Harper.