LAW ENFORCEMENT agencies can’t be expected to stop every terrorist attack, any more than they can prevent every mass shooting. If, as most investigators now appear to believe, brothers Tamerlan and Dzhokhar Tsarnaev acted on their own with a bomb design downloaded from the Internet, their plot posed a steep challenge to those agencies, including the FBI and Department of Homeland Security (DHS), responsible for detecting threats.

Nevertheless, there are reasons for concern about the two agencies’ performance, based on what is known so far about their tracking of Tamerlan Tsarnaev. The older and more radical brother was first identified as a possible extremist by Russia, which asked the FBI to investigate him in early 2011. Later that year, also after prompting from Russia, the CIA asked that his name be added to a watch list maintained by the National Counterterrorism Center, The Post’s Greg Miller and Sari Horowitz reported. His subsequent departure for Russia in early 2012 resulted in “a ping” to customs officials, Homeland Security Secretary Janet Napolitano told Congress on Tuesday. However, it appears the FBI never learned that Tamerlan had left the country and was not informed when he returned in July.

The FBI had concluded that the elder Tsarnaev posed no threat. It had reason to be skeptical about Russian warnings about emigre Chechens, many of whom are wrongly regarded by Moscow as terrorists, and Russian authorities did not respond to the bureau’s requests for additional information. That still leaves a couple of questions: Was there a breakdown in “critical information-sharing,” as Sen. Susan Collins (R-Maine) put it, that prevented the FBI and other agencies from learning about Tamerlan’s travel to Russia? And did the bureau know about the CIA’s communications with Russia?

No doubt Ms. Collins and others in Congress will press the question of whether “stovepiping” still exists. Such bureaucratic barriers to information-sharing were supposed to have been eliminated by post-2001 reforms — including the creation of DHS. But a deeper look at the FBI’s approach to counterterrorism would also be worthwhile. In addition to Tamerlan Tsarnaev, several people investigated by the FBI have gone on to commit attacks, including Abdulhakim Muhammad, who shot two soldiers at a Little Rock military recruiting office in 2009, and Maj. Nidal Hasan, who is accused of shooting and killing 13 people at Fort Hood later that year.

Meanwhile, the FBI has devoted considerable resources to sting operations against people it judges to be terror suspects, sometimes on what look like dubious grounds. Mother Jones magazine reported this week that, at the same time the FBI was concluding that Tamerlan Tsarnaev was not a threat in 2011, it launched an elaborate sting operation in Boston against Rezwan Ferdaus, who eventually pleaded guilty to charges of plotting to attack the U.S. Capitol with a remote-controlled model airplane loaded with grenades. In his case as well as others, it’s not clear that a sometimes far-fetched plot would have gone forward without the encouragement and help of FBI informants.

As we said, not every plot by terrorists can be detected. Respect for civil liberties means that not everyone who visits a jihadist Web site or is the subject of an inquiry by another government can be placed under permanent surveillance. But it’s worth asking whether the FBI’s methods for identifying and following up on threats need refinement.