– When the Obama administration imposed sanctions on individual Russians last month in response to Moscow’s armed intervention in Ukraine, one of the targets was a longtime part-owner of a commodities trading company called the Gunvor Group.

His name, Gennady N. Timchenko, meant little to most Americans, but buried in the Treasury Department announcement were a dozen words that President Obama and his team knew would not escape the attention of Russia’s president, Vladimir Putin. “Putin,” the statement said, “has investments in Gunvor and may have access to Gunvor funds.”

For years, the suspicion that Putin has a secret fortune has intrigued scholars, industry analysts, opposition figures, journalists and intelligence agencies but defied their efforts to uncover it. Numbers are thrown around suggesting that Putin may control $40 billion or even $70 billion, in theory making him the richest head of state in world history. For all the rumors and speculation, though, there has been little if any hard evidence, and Gunvor, for its part, has adamantly denied any financial ties to Putin.

But Obama’s response to the Ukraine crisis, while derided by critics as slow and weak, has reinvigorated a 15-year global hunt for Putin’s hidden wealth. Now, as the Obama administration prepares to announce another round of sanctions as early as Monday targeting Russians it considers part of Putin’s financial circle, it is sending a not-very-subtle message that it thinks it knows where the Russian leader has his money, and that he could ultimately be ­targeted directly or indirectly.

“It’s something that could be done that would send a very clear signal of taking the gloves off and not just dance around it,” said Juan Zarate, a White House counterterrorism adviser to President George W. Bush who helped pioneer the government’s modern financial campaign techniques to choke off terrorist money.

So far, the U.S. government has not imposed sanctions on Putin himself, and officials said they would not in the short term, reasoning that personally targeting a head of state would amount to a “nuclear” escalation, as several put it. But officials said they hoped to get Putin’s attention by targeting figures close to him.

“It’s like standing in a circle and all of a sudden everyone in the circle is getting a bomb thrown on them, and you get the message that it’s getting close,” said Sen. Robert Menendez, D-N.J., chairman of the Foreign Relations Committee, describing the way sanctions are getting closer to Putin.

Putin’s denials

Putin’s reported income for 2013 was just $102,000, according to a Kremlin statement this month. Over the years, he has crudely dismissed suggestions of personal wealth.

“I have seen some papers about this,” he said in 2008. “Just gossip that’s not worth discussing.”

How much Putin cares about money has long been a subject of debate both in Russia and in the West. On government payrolls since his days in the KGB, Putin to many seemed driven more by power and nationalism than by material gain. With access to government perks like palaces, planes and luxury cars, he seemingly has little need for personal wealth.

“If he really does have all that money salted away somewhere, why?” asked Bruce Misamore, who was chief financial officer of Yukos Oil before the Russian government imprisoned its top shareholder and seized its assets. “What good does it do him? Is it just ego?”

And yet, some have drawn attention to what appear to be expensive watches on his wrist and the construction of a seaside palace that the Kremlin denied was being built for Putin. Some argue that Putin may want money, or the appearance of it, because it is the measure of stature and power in a society whose transition to capitalism has produced instant billionaires.

Biggest and best

“I came to the conclusion after time that some of these reports may be seeded by people around Putin himself,” said Fiona Hill, who was the chief Russia expert at the National Intelligence Council and last year cowrote a book about Putin. “Russians have to have the biggest and the best.”

The Treasury Department has not provided evidence to back up its statement about Putin, but policy requires it to have enough verification to withstand a court challenge.

U.S. diplomatic cables obtained by the anti-secrecy organization WikiLeaks show sustained attention to the subject. The cables tied Putin not only to Gunvor but also to Surgutneftegaz, a large oil company, and even to Gazprom, the state-owned energy giant, but they used words like “rumored.” In one cable, for instance, diplomats cited a General Electric executive working in the region who privately estimated that the ­Russian leader was worth “well over $10 billion.”

The CIA in 2007 produced a secret assessment of Putin’s wealth that has never been released, according to officials who have read it. The assessment, the officials said, largely tracked with assertions later made publicly by a Russian political analyst who said Putin effectively controlled holdings in Gunvor, Gazprom and Surgutneftegaz that added up to about $40 billion at the time.

From the start of his political career, Putin has been dogged by suspicion. While he was deputy mayor of St. Petersburg in the 1990s, his office signed deals giving favored companies licenses to export $92 million in oil, timber and other products in exchange for an equal amount of imported food. The food never materialized.

Spared by his patron

Putin was not accused of personally benefiting, but a City Council committee led by Marina Salye recommended Putin’s dismissal for “incompetence” and “negligence and irresponsibility.” She also pushed for prosecutors to investigate. Putin blamed the companies involved and was spared by the mayor, Anatoly A. Sobchak, his political patron.

Still, it was not clear whether Putin in that era coveted money for himself or was more interested in deciding how it would be distributed as state assets were gobbled up by newly minted capitalists. In 2006, Bush kicked off an initiative targeting corrupt foreign leaders. Over the next year, his administration focused attention on learning more about the finances of leaders in the former Soviet Union.

The 2007 CIA assessment grew out of that. But different officials came away with different impressions of its reliability. Some said they considered it a reasonable appraisal of Putin’s worth based on solid reporting. Others said they considered it to be built largely on unsubstantiated talk.

Either way, the assessment roughly mirrored estimates made publicly at the end of that year by Stansilav Belkovsky, a Russian political analyst with ties to the Kremlin.

At least $40 billion

Belkovsky told European newspapers in 2007 that Putin had amassed a fortune of “at least” $40 billion through sizable shares of some of Russia’s largest energy companies.

“The reality is that Putin has others and entities to move money that he controls or that he might control ultimately,” said Zarate, the former Bush adviser. “The challenge with him is you don’t have an easy way of drawing the line to the assets he actually owns and controls currently. There’s a dimension of layering and relationships with people with whom he’s close and entities that serve as conduits that make it tricky to determine what is Putin’s and what is not.”

In the years since, others have looked at Putin’s finances.

In 2010, Sergei Kolesnikov, a businessman, published an open letter saying he had helped Putin secretly build a billion-dollar palace on the Black Sea. The Kremlin dismissed his claims as “absurd.”

In 2012, Boris Y. Nemtsov, an opposition leader, released a report detailing the presidential perks at Putin’s ­disposal, including 20 residences, 15 helicopters, four yachts and 43 aircraft.

Obstacles abound

But some hunting for Putin’s wealth have found obstacles.

In March, Cambridge University Press declined to publish a book by its longtime author Karen Dawisha, a Miami University professor, exploring how Putin built “a kleptocratic and authoritarian regime in Russia.”

The publisher wrote her saying it had “no reason to doubt the veracity” of her book, but deemed the risk of a lawsuit too high, according to letters published by the Economist. In a return letter, Dawisha called the decision “pre-emptive book burning.”

All of which makes the Treasury Department’s assertions last month so striking.

Garry Kasparov, the Russian chess master turned ­opposition leader, said Putin’s wealth must be so buried that it would be difficult to prove within the standards typically required by U.S. lawyers.

“I’m sure it’s reachable, but you might have to break some of the rules to reach it,” he said.

The sanctions issued so far, he said, have not made enough of an impression.

“They have to convince Putin that it will be serious,” he said.