Wang Qishan leads the Central Commission for Discipline Inspection, which has become Chinese President Xi Jinping’s main weapon in an anti-corruption campaign that has gone further than any other in the country’s modern history. (Feng Li/Getty Images)

No sign identifies the drab beige building off a busy thoroughfare in downtown Beijing. There is nothing to indicate that within its walls lies the most feared agency in China for members of the Communist Party.

The institution has an obscure name — the Central Commission for Discipline Inspection. But in the year and a half since Xi Jinping became China’s leader, it has become his main weapon in an anti-corruption campaign that has gone further than any other in the country’s modern history.

The campaign is meant to clean up the party’s image — so soiled by graft that some leaders fear public contempt could threaten their grip on power. It also appears aimed at consolidating Xi’s power. He has used the commission to weaken rival factions and, more broadly, to warn off anyone who might challenge his agenda.

(Read: How the secretive, powerful agency in charge of investigating corrupt officials works)

The anti-corruption agency has existed in some form since 1927. But for much of its history, its investigations were often seen as shams, used to justify the expulsion of low-ranking cadres or senior officials who had lost internal power struggles.

Now, its investigations have become far more aggressive and publicized, and the agency is taking on higher-ranking leaders. The commission is secretive and highly compartmentalized, but several current and former staffers and party officials talked about its operations on the condition of anonymity.

“There is no Western equivalent,” said Chris Johnson, the CIA’s former top China analyst. “It has elements of the FBI, U.S. Treasury Department, Secret Service and GAO [Government Accountability Office] all rolled into one.”

The Chinese commission has far more latitude than those U.S. agencies and is much more politicized. It operates entirely outside the legal system, as a Communist Party justice mechanism. Its investigators need no warrants to seize evidence. And it has the power to imprison and interrogate any party official.

Although the worst official punishment the commission can mete out is booting cadres from the party, its investigations are often transferred after the fact to the judicial branch, where the expelled officials usually receive heavy sentences.

The commission now appears poised to take down its biggest target yet: Zhou Yongkang, once a member of the top standing committee headed by Xi.

Some party members worry that the Zhou investigation, and other cases it might spawn, could wreck the balance between factions that has provided political stability to China.

“They are venturing further than they have ever gone before,” said one official, who previously worked with the commission. “It’s a delicate line they are trying to walk between this anti-corruption campaign and taking it so far within the party that it threatens to upset everything else.”

A different campaign

China’s one-party political system and its centrally planned economy have long provided fertile ground for graft. There are few checks on power such as an autonomous legal system, independent news media or legal political opposition.

Every Chinese leader in the past half-century has declared wars on corruption, toothless campaigns that often fizzled quickly.

Xi’s campaign has been different. Every week, new officials or executives of state-owned companies have been placed under investigation. The announcements often provide little detail, but bloggers and other citizens have rushed to fill in the blanks, exposing Chinese officials’ luxury villas and their many mistresses.

The commission’s most impressive feat, however, has been its takedowns of “tigers,” or high-level party officials. Last year, the agency investigated 31 officials at the vice-minister level or higher. In previous years, the number of such cases was not made public, but insiders say it rarely rose above 10.

Even those who believe that the anti-­corruption campaign’s real goal is to benefit Xi and strengthen the party’s image — not to clean up government — acknowledge it has reached further than any other in modern history.

One reason for that, according to party members, is the man Xi has put in charge of the campaign: Wang Qishan, often called the Communist Party’s “firefighter in chief.”

Whereas most officials ascend the party ladder by being cautious, Wang made his reputation by diving into crises, according to more than a dozen party officials and diplomats who have dealt with him. In the late 1990s, he guided Guangdong province through a debt disaster. After Beijing’s mayor was sacked in 2003 amid criticism that officials were covering up an outbreak of severe acute respiratory syndrome, or SARS, Wang was sent in. He was put in charge of the 2008 Olympic Games in Beijing and later the country’s financial sector.

Since his arrival at the anti-

corruption agency in late 2012 — when Xi took over the party — Wang has dramatically increased the number and impact of its investigations. He has added four inspection offices to the eight that already existed and diverted personnel from processing paperwork to assisting in investigations, according to agency officials.

Those who deal with him say Wang, 65, draws inspiration from a wide range of sources. He has urged advisers to read Alexis de Tocqueville’s “The Old Regime and the French Revolution,” a treatise on the excesses of the French aristocracy and its overthrow.

He is also a fan of the Netflix drama “House of Cards,” according to party officials, apparently impressed by the character Frank Underwood, a ruthless politician.

He has gone after cadres within the anti-corruption bureaucracy itself, setting up an office of internal investigations and ordering employees to refuse gifts.

“To forge iron, you yourself must be strong,” he said in a meeting last year, according to state media.

Wang clearly has Xi’s backing, experts say. “There’s a high degree of trust between Xi and Wang. It’s a tight relationship and collaboration,” said one Western diplomat who has interacted with both men.

Their bond can be traced as far back as the early 1970s, when, like many urban youths, they were sent to the countryside under a program aimed at warding off “bourgeois” tendencies. Both worked near the city of Yan’an, and during one visit by Xi to Wang’s home, the two even shared a quilt as they slept, according to Zhang Siming, a Yan’an writer who interviewed Xi in 2001.

Secretive process

With corruption so rampant in China, the question often isn’t who is dirty, but whom to target. And many say that the decision remains a political one.

Smaller provincial disciplinary commissions are often tasked by local party bosses with pursuing lower-ranking officials. The central commission in Beijing, however, is the only one allowed to investigate officials with the rank of vice minister or higher.

Little is known about how decisions are made in the most divisive cases, such as those involving Bo Xilai, a party official purged in 2012, and Zhou, China’s former security czar. But two officials who have worked with the central commission said sensitive cases need the approval of the party’s politburo and, in some cases, the seven-member standing committee.

Having solid evidence can be crucial in rallying support for a case, or stopping a target’s patron from saving him.

The Zhou investigation has dragged on for months in part because of political maneuvering, many officials say. The agency’s investigators have painstakingly built a case against Zhou by taking down his subordinates one by one. Even now, however, few people know what the outcome will be.

Investigators need to be savvy, since they can find themselves pulling on threads that can lead to top party leaders and industries they have no clearance to probe.

In addition, investigators are often operating on turf where local Communist leaders control every aspect of government.

One longtime agency official said a successful investigation requires cutting off such leaders from their networks of allies and curbing their ability to hide evidence and silence subordinates.

He compared the process to forcing someone to get off the toilet. “Until you push their butt off the seat, you cannot uncover the true stench of their corruption,” he said.

Increasingly important to the central commission are skills such as forensic accounting, to trace ill-gotten assets, and the ability to mine data on seized cellphones and computers.

Power of interrogation

Despite the veneer of sophistication in the investigations, however, the tool investigators rely on most remains interrogation. They have the power to spirit away any party member for months at a time for questioning — a much-feared process known as shuanggui. (Literally, the word means “dual designation” and refers to the party’s power to designate a time and place for questioning its officials.)

Little is known about the nature of the detention at the central level. There is much more information about what happens in the provincial discipline committees. In public accounts, some officials at the provincial level have recounted being beaten, burned with cigarettes, deprived of sleep and subjected to simulated drowning. Many have committed suicide or died under shuanggui, according to officials.

The interrogations carried out by the national anti-corruption agency are less likely to involve the use of physical pain, said officials who have worked with the agency, but rough tactics aren’t unheard of.

Investigators for the commission can detain officials for up to six months, said Ren Jianming, an anti-corruption expert who is an adviser to the agency. But their superiors can extend the period.

Detained officials are watched at all times by teams of six to eight investigators, their only human contact other than interrogators.

It’s hard, of course, to feel sorry for China’s corrupt officials. But some activists, including a few who have suffered at the hands of those very officials, point out that investigating and punishing them this way violates human rights and rule of law. Experts also say such efforts fail to address the root causes of corruption in the system, like lack of transparency and independent oversight over the party.

But in recent months, Xi has signaled that he only intends to hand more power to the national agency.

For years, local party bosses have managed to escape corruption probes because they appoint and pay investigators on local anti-­corruption committees. Under Xi’s new policies, those local agencies will soon report directly to Beijing’s central commission, although their investigators’ salaries will still be paid locally.

“Previous leaders have been trying to do this for years. It’s a big deal if they’re successful,” said former CIA analyst Johnson, now at the Center for Strategic and International Studies, a Washington-based think tank. “Ultimately, it means more power, more control. Anti-corruption is just a means to their ends.”

Li Qi, Guo Chen and Xu Yangjingjing contributed to this report.