After identifying the top three to four aircraft, we begin a deeper investigation, which takes a much closer look at aircraft condition and verification of all specifications.

FORBES: What's The Best Jet For You? (Part 3)

The FBI receives thousands of such requests from foreign agencies and unless it finds cause for deeper investigation, agents aren't authorized to conduct surveillance or otherwise track people, the official said.

WSJ: Trip to Russia Focus of Boston Bombing Probe

It is unclear whether the FBI would have done a deeper investigation of Tamerlan had it known the details of the Russian wiretaps, the U.S. law-enforcement official said, and whether a further probe would have turned up anything more suspicious.

WSJ: Officials Seek to Question Suspect's Widow

Democrats, emboldened, have called for deeper probes (last month, Republicans headed off an additional investigation into WMD-related contacts between the vice-president's office and the CIA, and another into whether America co-operated sufficiently with United Nations weapons inspectors before the war).

ECONOMIST: Who's hiding what? | The