McKinsey & Co is the biggest name in the consulting business. Established in 1926, it employs 30,000 people, maintains offices in more than 130 locations and counts Pete Buttigieg, the US transportation secretary, among its alumni. From vaping to non-profits, insurance to energy, government work to healthcare, the McKinsey thumbprint is there.

Traditionally, McKinsey possessed the luxury of distance, watching from the sidelines as clients bore the brunt of scrutiny, lawsuits and risk. But the space between field and bleachers has narrowed. McKinsey finds itself under the microscope.

When McKinsey Comes to Town is highly informed, a fascinating read. The authors, New York Times investigative reporters Walt Bogdanich and Michael Forsythe, have done their homework. They name names, connect dots and unearth documents. Sources speak in Technicolor.

Bogdanich is a three-time Pulitzer winner. Forsythe brings a keen eye to the intersection of money, politics and China. He was previously based in Hong Kong. As it happens, McKinsey has worked for both the US defense department and for Chinese state-owned companies that have aided Beijing’s military buildup.

McKinsey says Bogdanich and Forsythe “fundamentally misrepresent our firm and our work”. It issued a similar statement when the Times and ProPublica highlighted its remit on behalf of US Immigration and Customs Enforcement and US Customs and Border Protection. ProPublica said: “McKinsey Called Our Story About Its ICE Contract False. It’s Not.” The contract with Ice was reportedly worth $18m.

McKinsey has advised more than 40 US agencies. It played an outsized role in Jared Kushner’s attempts to cope with Covid, which originated in China. At the same time, it maintained a presence in China. The apparent conflict of interest triggered congressional concern. A group of Republicans claimed McKinsey’s work “on behalf of Chinese … firms, is tantamount to work on behalf of the CCP [Chinese Communist Party] and could lead to direct or indirect support for the CCP’s armed wing, the People’s Liberation Army”.

Amid rising tension between Washington and Beijing, McKinsey’s connections, contracts and loyalties will probably continue to draw attention.

The firm remains in the news. In February 2021, McKinsey entered into nearly $600m of legal settlements with state attorneys general. Why? The platinum-plated powerhouse purportedly helped Perdue Pharma “turbocharge” opioid sales. Plaintiffs alleged that “McKinsey sold its ideas to … Purdue Pharma …from 2004 to 2019, including before and after Purdue’s 2007 guilty plea for felony misbranding.”

McKinsey also counts the US Food and Drug Administration as a client. But that’s just the beginning. To quote members of Congress, on at least four occasions the company may “have passed along non-public information based on its relationship with the FDA or discussed its willingness to do so” with Purdue Pharma.

Bogdanich asks: “What does that mean when you have an opioid manufacturer who’s pushing opioids in the middle of an epidemic?”

Since 1999, opioid-related deaths have risen more than fivefold. In two decades, opioids have killed more than 450,000 in the US. Life expectancy is down and it’s not just because of Covid. Death by despair is rising.

In 2020, McKinsey apologized for its involvement with Purdue Pharma, “recogniz[ing] that we did not adequately acknowledge the epidemic unfolding in our communities or the terrible impact of opioid misuse”.

McKinsey also counted as a client Juul Labs – the vaping company and scourge of teachers, moms and dads – billing it between $15m and $17m. Its most important work for Juul involved responding to an FDA crackdown on youth vaping.

Youthful addiction can be profitable – until it isn’t. In September, Juul and more than 30 state attorneys general reached a $438.5m settlement. The e-cigarette manufacture did not admit culpability. McKinsey was not involved in the settlement. Juul hovers on the cusp of bankruptcy.

Bogdanich and Forsythe focus on another “long-standing” McKinsey policy – simultaneously serving competing clients with “conflicting interests” as well as “counter-parties in merger, acquisition and alliance opportunities”. In plain English, McKinsey can find itself on both sides of transactions.

Self-policing works – until it doesn’t. Unlike the strictures that govern lawyers, the rules that pertain to consultants, if any, are porous and less rigid. Last month, South African prosecutors indicted McKinsey on unspecified charges related to the alleged looting of Transnet, the state freight rail monopoly.

“We believe the charges filed against our South Africa office are meritless and we will defend against them,” a McKinsey spokesman responded.

Regardless of the outcome of the case, McKinsey’s experience in South Africa stands as a study in the perils posed when governments offload government functions to non-state actors.

McKinsey will face continued scrutiny. Then again, it is unclear if such work as that of Bogdanich and Forsythe can or will lead to change. McKinsey services remain in demand. Eager college and graduate business school students line up for a shot at snagging the brass ring.

Speaking to Bogdanich and Forsythe, one former McKinsey consultant put the reach of the firm into some perspective. Forget secret cabals, “illuminati, lizard people, or globalists” he said. Instead, “there is … McKinsey”.

• When McKinsey Comes to Town: The Hidden Influence of the World’s Most Powerful Consulting Firm is published in the USby Penguin Random House