The Boston Marathon bombings could provide an opportunity for the U.S. and Russia to find some common ground for cooperation as authorities investigate the two ethnic Chechens accused of carrying out the attack.
Ties between the two nations have soured over disputes about stopping Syria's civil war, child adoptions and other issues. But understanding how the brothers became radicalized is of paramount importance to Washington as it seeks to prevent similar attacks. It's also important to Moscow, which long has battled terrorism in its southern territories.
But the tragedy also risks hardening resentment between former Cold War foes which, under President Barack Obama and Prime Minister Vladimir Putin, have seen efforts to "reset" relations falter. Even their counterterrorism coordination has sometimes been strained.
"Certainly, this incident is going to lead both sides to re-examine the issue" of intelligence sharing, said Andrew Kuchins, a Russia analyst at the Center for Strategic and International Studies. "It's not hard for Russia to imagine that these two brothers who became radicalized in the United States could have been inspired to go back to Russia to launch an attack."
Much depends on how the U.S. and Russian governments mobilize the emotions produced by Boston a week after one of the most significant terrorist attacks on U.S. soil since 9/11.
The American public is demanding quick answers. In Russia, one prominent official already has declared the twin bombings an American problem. And even if U.S. or Russian authorities never link the attack to Chechen extremist groups, some fear the Kremlin nevertheless will use that as added justification for a harsher crackdown on the Muslim-majority region, especially as it prepares to welcome the world to the 2014 Winter Olympics.
"Both sides seek to mend ties," says Anya Schmemann at the Council on Foreign Relations. "But it remains to be seen if the Boston bombings offer an opportunity for U.S.-Russian cooperation or if it will lead to an overly aggressive Russian response in the North Caucasus that could be worrisome for the United States."
Investigators in the United States are trying to figure out how Tamerlan Tsarnaev, 26, and his 19-year-old brother Dzhokhar evolved from asylum seekers to apparently assimilated U.S. residents to alleged terrorist bombers.
Tamerlan was killed during a furious getaway attempt last week after the bombings that killed three people and left more than 180 injured. Dzhokhar is hospitalized in serious condition with a neck wound and has been charged with crimes that could lead to the death penalty.
So far, no definitive answer on the Tsarnaevs' motivations has been provided, but the U.S. says it is coordinating closely with Russia.
Obama and Putin spoke last week by telephone and the American leader praised the "close cooperation that the United States has received from Russia on counterterrorism in the wake of the Boston attack," according to a White House statement.
The issue is likely to be high on the agenda Tuesday when Secretary of State John Kerry - a Boston native - meets Russian Foreign Minister Sergey Lavrov in Brussels.
Obama's national security adviser, Tom Donilon, was also just recently in Moscow for talks on a range of bilateral problems.
On Syria, Russia backs President Bashar Assad's regime while Washington supports the rebels trying to oust him. The countries are bickering over Putin's crackdown on civil society groups and his order to halt any future American adoptions of Russian children. U.S. missile defense plans in Europe are also a sore point.
A modest improvement in counterterrorism cooperation is unlikely to fundamentally change these splits. But it could at least halt a slide in relations that has left few in the Obama administration still touting the benefits of its much-hyped "reset" of four years ago.
Regarding Boston, the Russian government will only be too happy if the trail leads to Chechen militant groups it has blamed for far deadlier terrorist attacks in Russia over the last decade and a half.
They've served as Putin's primary rallying cry for crackdowns in the Russia's restive South, to which the U.S. has responded ambivalently. Any connection to an Islamist separatist group would dovetail with Russia's oft-repeated argument that Syria's rebels ought to be feared and that the Assad regime's collapse would risk greater international terrorism.
Chechnya's conflict began with a separatist war in the 1990s after the Soviet Union fell apart, morphing into an Islamist insurgency that Putin brutally suppressed a decade later. An estimated 100,000 people were killed.
The toll was high for Russians as well: apartment explosions in Moscow that prompted Russia's second invasion in 1999; an attack on a Moscow theater three years later that left 129 hostages dead; and a raid on the town of Beslan in 2004 that ended with 330 people, about half children, killed. In recent years, suicide bombings killed dozens on Moscow's subway and at its airport. A train bombing claimed 26 lives.
The United States has backed Russia's battle against terrorist groups, but has balked at the heavy-handed tactics that have caused widespread woe among civilians.
Human rights groups cite ongoing atrocities from assassinations to indiscriminate arrests based on ethnicity and say the Kremlin too often has lumped all separatists together under al-Qaida's umbrella. This has complicated U.S.-Russian counterterrorism cooperation.
"Putin has been making the point for 14 years that Chechnya affects all of us," said Fiona Hill, formerly the White House's national intelligence officer for Russia under Obama and President George W. Bush.
But, she said, Russia seldom showed interest in threats specific to the United States. "That's always been the frustration of our counterterrorism officials - that the Russians have always wanted us to focus on their issue."
Boston's aftermath offers a clear opportunity for better intelligence cooperation, especially if the U.S. determines that the FBI let one of the bombers slip through its fingers.
Both the U.S. and Russia want to know what the older Tsarnaev did in southern Russia for six months last year. Family members say his stay had nothing to do with separatist or Islamist rebels, but he was already apparently on Russian intelligence's radar. They asked that the U.S. question him in 2011.
U.S. authorities found nothing that sparked their interest in Tsarnaev and stopped watching him.