The letter was written in Cyrillic script and passed by hand from Russia’s security service to the FBI in Moscow on March 4, 2011.

It barely covered a single page and contained a vague warning that a mother and her son, both Russian immigrants to the United States, had turned to radical Islam. Tamerlan Tsarnaev, then 24, was described in the text as a “militant” who could be headed to Russia to link up with unidentified “underground” groups.

The tip, like a struck pinball, ricocheted around the vast U.S. counterterrorism system for 16 months. It pinged against agencies including the CIA, the Department of Homeland Security and the National Counterterrorism Center. It generated entries on databases with such abbreviations as TIDE, TSDB and TECS.

Then, at the worst moment, the blinking lights went dark. The last time Tsarnaev surfaced as even a blip on U.S. screens was when he returned from Russia this past July. That was before he began assembling an online library of jihadist videos, before his trip to a fireworks shop in New Hampshire for gunpowder, before he and his brother, Dzhokhar, made their way toward the crowded Boston Marathon finish line with bulging backpacks.

It has been more than a decade since the United States began building its massive counterterrorism infrastructure, an apparatus that has been reconfigured several times in recent years after a series of near-miss attacks.

The strike in Boston marked the first time that a terrorist bomb plot slipped past those elaborate defenses and ended in casualties in the United States. Whether that outcome represents an intelligence failure is already the focus of a multi-agency review as well as a heated political debate.

The details that have emerged so far suggest there are still institutional gaps that could be fixed to bolster the nation’s counterterrorism system. But the bombings also exposed a less-reassuring reality: Even when defenses function as designed, they can be undermined by factors beyond their control.

In Boston, some of those factors were as fundamental and elusive as timing and luck.

“When this happens, there’s sort of an automatic response to find a linkage to failure,” said Andrew Liepman, who served as deputy director of the National Counterterrorism Center until last year. It’s perfectly reasonable to look into whether there were breakdowns, Liepman said. “But that massive counterterrorism infrastructure works amazingly well to protect the country. We need to get used to the idea that it isn’t foolproof.”

In the case of Tamerlan Tsarnaev, the obstacles for U.S. authorities ranged from a misspelling on an airplane boarding pass to the apparent refusal of Russian authorities to go beyond their initial tip. Ultimately, however, perhaps the best chance to detect and disrupt the plot fell into a six-month span on the calendar, the near-empty space between when the FBI stopped watching Tsarnaev and when he is alleged to have begun laying the groundwork for the Boston plot.

President Obama said last week that the FBI and other agencies had made no missteps “based on what I’ve seen so far.” He pledged a review to determine whether those agencies could have done anything differently, but he warned that vulnerabilities will persist.

“This is hard stuff,” Obama said. “One of the dangers that we now face are self-radicalized individuals who are already here in the United States, in some cases may not be part of any kind of network. . . . And those are in some way more difficult to prevent.”

Tsarnaev was killed in a shootout with police four days after two homemade bombs tore through the crowd at the marathon on April 15. His younger brother was captured later the same day and faces federal charges that carry the death penalty. Dzhokhar, 19, told the FBI after his capture that he and his brother were motivated by anger at the United States over the wars in Iraq and Afghanistan, according to U.S. officials.

The initial tip about Tamerlan Tsarnaev, flawed and fragmentary, was an early indication of how the pursuit of him would unfold. What follows is a reconstruction of that trail, based on interviews with current and former officials of multiple agencies in the United States. Some of those people spoke on the condition of anonymity to speak freely about the ongoing investigation.

That first document contained two possible dates of birth for Tsarnaev, both of them wrong. The letter had two versions of his name in Cyrillic, a discrepancy that was multiplied when the FBI and CIA came up with competing transliterations.

Within days, the tip was relayed from Moscow to the Joint Terrorism Task Force in Boston, part of a national network of FBI-led, multi-agency cells set up to foster collaboration among federal, state and local agencies.

Using immigration records and other databases, the task force found the proper spelling of Tsarnaev’s name and the correct date of birth. On March 22, 2011, the FBI opened what is known as an “assessment” of Tsarnaev. This is a low-level inquiry that the bureau employs when it has insufficient grounds for a criminal investigation.

The FBI questioned Tsarnaev and his mother and set up surveillance outside his apartment in Cambridge, Mass. A U.S. official briefed on the matter said the bureau never entered the dwelling but “did some light, intermittent surveillance. Less than full-scale surveillance, but more than driving by the house.”

The effort, according to U.S. officials, yielded nothing to substantiate Russia’s suspicions. On June 24, 2011, officials said, the FBI closed its assessment without adding a single “derogatory” note to Tsarnaev’s file, though his name remained in a broad FBI database called Guardian.

The FBI’s scrutiny was modest compared to its aggressive approach in dozens of other terrorism probes. The bureau has relied extensively on informants posing as Islamist militants to root out would-be terrorists. In many cases, the informants delivered seemingly real explosives to unwitting suspects before authorities moved in.

Those tactics led to 41 arrests between 2009 and 2011, according to the Center on Law and Security at New York University School of Law. They also have led some civil liberties advocates to criticize the bureau as overzealous.

But U.S. officials said the FBI never considered using an informant to test Tsarnaev. He didn’t meet the threshold, which requires evidence that someone is already contemplating violence. Asked whether Tsarnaev was a sting candidate, a U.S. law enforcement official replied, “We didn’t get close to that.”

Even so, the FBI’s evaluation of Tsarnaev was more thorough than the cursory look it had taken at the last suspect with terrorism ties who killed people in the United States.

In 2009, two separate joint terrorist offices developed alarming evidence that U.S. Army Maj. Nidal Hasan was espousing radical religious views and corresponding by e-mail with Anwar al-Awlaki, a known al-Qaeda figure in Yemen. But FBI agents in San Diego and Washington clashed over whether Hasan posed a real danger. According to a 2011 Senate report, the officials never questioned Hasan before he was accused of killing 13 people in a shooting spree at Fort Hood in Texas.

In Tsarnaev’s case, even after the FBI closed out its assessment, warnings about him continued to rattle around other corners of the U.S. counterterrorism system.

In September 2011, the Russian Federal Security Service, known as the FSB, hand-delivered an apparently identical copy of its Tsarnaev letter to the CIA station in Moscow. The spy agency relayed the information to the National Counterterrorism Center, which has the lead role in running the watch lists that are supposed to keep suspected extremists at bay.

As a result, Tsarnaev’s name appeared the next month in the largest of the cascading counterterrorism data pools that track potential threats.

He was placed initially on the Terrorist Identities Datamart Environment, a massive repository of the names of virtually anyone who has come under suspicion for ties to extremism in the United States or abroad — a list that would swell to 875,000 identities by the end of 2012.

He was also added to the Terrorist Screening Database, known as TSDB, which is a roster of roughly 500,000 names administered by a unit of the FBI.

In one of the more confounding aspects of the case, Tsarnaev was not added to the database by the FBI, which had taken the closest look at him. U.S. officials said the bureau puts a person in the database only when an “assessment” advances to the more serious level of “preliminary investigation.”

The CIA and the National Counterterrorism Center engineered his addition to the list, although neither agency had the authority to investigate him in the United States.

Ultimately, the decision hardly mattered. U.S. officials said Tsarnaev’s entry in the TSDB initiated no additional scrutiny, and he was never placed on more serious registries, which designate travelers for additional screening at airports and, in the most serious cases, put them on a “no-fly” list. Those are more selective databases, each of which contains about 20,000 names.

Still, his name was in the system, and Tsarnaev came to authorities’ attention again in January 2012, three days before he left for Russia. His family traces its origins to the southern Russian provinces where Islamic insurgents have challenged Moscow for years.

Ten months earlier, when the FBI had added Tsarnaev to the Guardian database after the Russian tip, a U.S. Customs and Border Protection officer assigned to the Boston terrorism task force had attached a “lookout” to his name in yet another database, this one used to track suspicious travelers as they exit or enter the United States.

The system, called the Treasury Enforcement Communication System and known as TECS, is designed to notify authorities three days before a person is scheduled to fly, with a second notice the day of departure or arrival. But the second notification never came for Tsarnaev. When he checked in for his Aeroflot flight to Moscow, his name “was inputted manually” by an airline employee and “spelled wrong,” said a U.S. official.

Tsarnaev’s ensuing six-month trip substantiated at least part of the initial Russian warning — he was headed back to his home country. But while Russia had first raised suspicion about Tsarnaev, U.S. officials said that his ties to that country and the troubled U.S.-Russia relationship reduced the likelihood that he would be seen as a threat by the Americans.

From the outset, FBI officials were wary of Russia’s motivation, convinced that Moscow had often used unfounded allegations to try to harass expatriates. That suspicion only grew, officials said, when Russia failed to respond to FBI requests for more on Tsarnaev in the earliest stages.

The two sides have made hundreds of requests of one another in recent years, but are still plagued by Cold War mistrust, said Jim Treacy, a retired FBI agent who served as legal attache, the bureau’s top embassy position, in Moscow between 2007 and 2009.

“They were standoffish, suspicious, not openhanded,” Treacy said. Rebuffed requests for more information were interpreted in two ways. “If they didn’t provide anything, the assumption was [their information was obtained through] a technical nature that they don’t wish to disclose, or there is some other reason that they actually came forward.”

More broadly, Russia’s bastions of Islamist extremism — including Dagestan, which Tsarnaev visited — were not seen as exporters of violence to the United States, unlike Pakistan or Yemen.

Russia’s tip emphasized the threat Tsarnaev allegedly posed to that country. The fact that he traveled to Russia with little interference from authorities there has been cited by U.S. officials since the Boston bombings as proof that Moscow had no solid evidence on Tsarnaev.

The FBI has sent agents to Russia to pore over Tsarnaev’s trip amid reports that he may have met with alleged militants such as William Plotnikov, a Canadian citizen of Russian origin who had joined an insurgent group in Dagestan. Plotnikov was killed in a raid by Russian authorities toward the end of Tsarnaev’s trip.

U.S. officials now believe that the Boston bombing plot was set in motion after Tsarnaev’s return to Cambridge. He and his brother are suspected of assembling a half-dozen explosive devices following recipes they researched online — the kind of activity that might have been detected when the FBI was paying its visits to Tsarnaev the previous year.

When Tsarnaev arrived at John F. Kennedy International Airport in New York on July 12, 2012, he faced no additional screening or questioning, although the “lookout” in the TECS system triggered an alert to the customs officer in the Boston terrorism task force.

There is no indication that the officer shared that information with others in the cell, including the FBI.

U.S. officials have said the customs officer might have mentioned the notification in passing, without leaving a record, but they declined to say whether the officer has said he did so.

At that point, there was no new information from Russian authorities. The FBI’s assessment from a year earlier remained closed. Even if the customs officer had passed along the travel alert, the bombing that killed three people and injured more than 260 might not have been averted.

“I don’t think that would have tripped anything,” said an FBI official.

Julie Tate, Peter Finn and Ellen Nakashima contributed to this report.