WASHINGTON – The team led by special counsel Robert Mueller failed to do everything it could to determine what happened in the 2016 election, shying away from steps like subpoenaing President Donald Trump and scrutinizing his finances out of fear that he would fire them, one of Mueller's top lieutenants argued in a new book that serves as the first insider account.

"Had we used all available tools to uncover the truth, undeterred by the onslaught of the president's unique powers to undermine our efforts?" wrote former prosecutor Andrew J. Weissmann, adding, "I know the hard answer to that simple question: We could have done more."

Weissmann sharply criticized the president as "lawless" but also accused Mueller's deputy, Aaron Zebley, of being overly cautious, according to an account in the Atlantic of the book, "Where Law Ends: Inside the Mueller Investigation," which Random House will publish next week. Random House provided an early copy to the New York Times.

Previously a longtime lawyer at the FBI for Mueller, who was the bureau's director for 12 years, Weissmann ran one of three major units for the special counsel's office: "Team M," which prosecuted Trump's former campaign chairman Paul Manafort for numerous financial crimes.

Manafort had worked for pro-Russian interests in Ukraine, and the investigation uncovered ties by his business partner, Konstantin Kilimnik, to Russian intelligence. The Mueller team learned that Manafort had shared internal campaign polling data with Kilimnik, who in turn sought Trump's approval for a plan by which all of eastern Ukraine would be broken off and Russia would essentially take over the region.

Weissmann was scathing about Trump and his allies. He called the president "lawless" and "like an animal, clawing at the world with no concept of right and wrong." He accused Attorney General William Barr, Mueller's old friend, of having "betrayed both friend and country." Barr used his early access to the special counsel report to warp public perception of it — including declaring Trump exonerated of obstruction of justice, when the report recounted numerous episodes in which Trump tried to impede the inquiry.

While Weissmann wrote with affection about Mueller, he also portrayed his boss as being excessively diffident. Even when the investigation was wrapping up, reducing the risk that they would be prematurely fired, Mueller held back by not clearly stating in the report that Trump obstructed justice — which later gave Barr his opening to put a more positive spin on the team's findings.