WASHINGTON (AFP) - US efforts to track potential terrorists are falling short, experts say, pointing to a jumble of overlapping watch lists and miscues that let one of the Boston bombing suspects slip through the cracks despite warnings about him.
Tamerlan Tsarnaev, the older of two brothers suspected of planting bombs near the marathon finish line, had drawn the attention of authorities more than once. Russian officials warned their US counterparts of their concern, and the CIA asked the top US counterterrorism agency to add Tsarnaev to a terror watchlist.
Yet, when he left for Russia in 2012 to a region known for Islamic insurgent stirrings, his trip was flagged by the Department of Homeland Security, but not by the FBI.
And his return to the US passed did not ring any alarm bells.
"The system didn't work," said Mr Christian Beckner, deputy director of George Washington University's Homeland Security Policy Institute, "maybe because of a different spelling of his name." "That was a real missed opportunity because of a complicated system," he lamented.
Mr Michael German, a former FBI agent who is now a terrorist expert at the American Civil Liberties Union, agreed: "The fact that multiple agencies keep these lists, different lists with different populations, is a huge problem."
Further muddying the waters, he added, "the lists are so blended with so many innocent people and irrelevant information, they're no longer reliable".
The main list, compiled by the National Counterterrorism Center, is called the Terrorist Identities Datamart Environment, or Tide. It contains a half-million names.
"I don't think anybody really supposes that there's half a million terrorists," German said.
The FBI also has a monitoring list, as does the Department of Homeland Security and US Customs, not to mention the "No Fly List" that bars certain individuals from boarding any aircraft.
"If there's any suspicion, people end up on the list, often without real factual basis," the ACLU analyst emphasised.
"If you just happen to have a name similar to a real terrorist on the list, that will cause the US government to focus on you and divert resources away from real threats."
And then there are the various aliases, double identities and different spellings of names from different alphabets, which complicate the matter and lead to mistakes.
Despite efforts to standardise the system after the 9/11 terror attacks, "the principal problem is lack of coordination between different agencies," said Jeffrey Addicott, director of the center for terrorism law at St Mary's University in Texas.
"This is just human nature: different agencies compete for information with each other, there's a human jealousy, there's a 'I got it first'" mentality, he said.
He recalled the case of Umar Faruk Abdulmutallab, the so-called "underwear bomber", who tried to launch an attack on a US-bound airplane on Christmas in 2009.
"His dad went to the State Department and complained about his son being a potential terrorist, and this information was never passed on to the other agencies," Mr Addicott said.
The FBI, the DHS and the National Counterterrorism Center each have different criteria for deciding to put a person under surveillance - criteria that are subjective and undisclosed, he added.
"An agency might think you're a potential threat, another might say no, no, it's just free speech," he explained.
In 2010, a report by the Inspector General - tasked with detecting and deterring waste, abuse and fraud in the Justice Department - found that the FBI list included political activists who had never posed any kind of threat.
The report said the FBI didn't add names "at the appropriate time, and they don't take people out of the lists once they've been cleared from investigation", ex-agent Mr German said.
But the real challenge, according to Mr Addicott, it's that the US lacks something standard in many countries, including France: a national identity card. "That's why it's so hard to track people. Without it, it's an impossible task."