It was meant as a post-9/11 reform. The TIDE terror list was established to be the federal government’s central repository for information about suspected or actual terrorists who could pose a threat to the United States. TIDE, an acronym for the clumsily named Terrorist Identities Datamart Environment, was part of a government overhaul to end the kind of bureaucratic stovepiping and communications failures that became evident after the attacks. At the time, there were as many as a dozen separate watch lists strewn across the government, many of which were not accessible to the very federal agencies charged with defending the country against terrorism. Consolidating the data into a master list, officials argued, would minimize the chances that potential terrorists could slip through the cracks.

But the Boston Marathon case illustrates the limitations of terror watch lists in a democracy where keeping tabs on potential terrorists must be balanced against the civil liberties of citizens. Moreover, in some ways the establishment of the massive, unwieldy list has created other problems that work at cross purposes with its original objective.

Reuters has reported that Tamerlan Tsarnaev, the older of the two brothers implicated in last week’s bombings, had been listed in the TIDE system as far back as 2011. Initially, Russian authorities asked the FBI to look into him. They suspected he’d become a radicalized Islamist and feared he might turn to violence in Russia. But an FBI investigation, including interviews with Tamerlan and some of his relatives, turned up nothing to support the Russians’ claim. On multiple occasions after that, the FBI sought additional information from the Russians but never heard back. Absent more evidence, officials say, agents were barred from using more intrusive investigative techniques like wiretaps or undercover informants. Then, in August 2011, the FSB, Russia’s state security service, made a nearly identical request of the CIA, which ran its traps on Tamerlan but also came up empty. The brief episode prompted the agency to “nominate” Tamerlan for inclusion on the TIDE list. His inclusion in the database, which is overseen by the National Counter-Terrorism Center, is prompting questions about whether he should have been more prominent on the FBI’s radar screen—especially after returning from a six-month trip to his native Russia in 2012.

But what exactly is the role of the TIDE list and what are its real implications for catching terrorists? The massive database contains some 700,000 people, though a significant portion may be duplicates because of variations in the spellings of names. Officials say the bar for placement on the list is low—technically, “reasonable suspicion,” a squishy threshold that could include, for example, the un-vetted assertions of a “walk-in” at a U.S. embassy. Undoubtedly a high percentage of those on the list are not terrorists themselves or have no connection to terrorism. In any event, the U.S. government does not have anywhere near the resources to investigate more than a half million suspects, never mind the civil liberties consequences of probing on such a massive scale.

“It’s not a database of guaranteed terrorists,” says Philip Mudd, a former counterterrorism official at both the FBI and the CIA.

Which begs the question: why even have such a large list of potential terrorists? For one thing, there are much smaller and more selective lists that are derived from the TIDE master list. They include the Transportation Security Administration’s Selectee List, which targets passengers for additional screening, and the TSA’s No-Fly List, which bars passengers from boarding commercial planes for travel in or out of the United States. But the TIDE list serves other important purposes, according to U.S. officials, including the synthesis and integration of data that is crucial to intelligence analysis. “It affords us the opportunity to make connections through disparate pieces of [intelligence] collection over an extended period of time,” says a U.S. counterterrorism official who declined to be identified. And there are major intelligence sharing benefits as well. One key example: when an agency comes across new information about an individual already on the list, they add it to the central repository, ensuring that the information is shared across the federal government.

In some ways, grasping the value of the TIDE list begins with understanding what it is and what it is not. “It’s not a database of guaranteed terrorists,” says Philip Mudd, a former counterterrorism official at both the FBI and the CIA. “But if we have something that at a low level hits a trip wire, shouldn’t we have a repository? I would say yes, so that we can tell the police in Oklahoma City that the guy you picked up for speeding is worth a second look.”

Of course, maintaining such a voluminous list presented challenges early on—among them, a high number of false positives because of confusion about names. Some years ago, the wife of former Alaska Senator Ted Stevens was periodically delayed at airports while security officials sorted out whether she was actually Cat Stevens, the British songwriter who changed his name to Yusuf Islam and was for a while on the terrorist watch list. But officials say the NCTC has worked out the kinks in the TIDE system and has reduced the number of mistaken hits substantially.