Disturbing information has accumulated in recent days about gaps in U.S. counterterrorism vigilance that may have allowed the Tsarnaev brothers to escape notice as they allegedly prepared bombs detonated at the Boston marathon that killed three spectators and wounded close to 200 others.

Russian authorities in 2010 and 2011 twice warned the U.S. through both the FBI and the CIA that they suspected the elder brother, Tamerlan, and his mother of being connected to Islamist terrorists in Russia. Their names were placed on watch-lists maintained by the National Counterterrorism Center. A Customs official at the Boston-area counterterrorism center was notified of Tamerlan’s plans in 2012 to travel to Dagestan, a Russian area where Islamist groups operate, but there was no follow-up.

Apparently, travel to terror-plagued areas of Russia is not considered a warning sign. Had the elder Tsarnaev traveled instead to Pakistan or Yemen, officials have said, it is more likely that the FBI would have followed him more closely and attempted to intercept his communications.

Adding to the significance of the 2012 trip, officials have determined that at least one of the Boston Marathon bombs was detonated by remote controls such as used in model aircraft. This has led some experts to ask whether Tamerlan Tsarnaev had been given instructions on bomb making while in Russia.

Commenting on these developments, Sen. Lindsey Graham, R-S.C., said Thursday that the Boston bombing case “is becoming, to me, a case study in system failure.”

“You have Russian intelligence services contacting two agencies within our federal government responsible for our national security, the FBI and the CIA,” he said. “They tell us, ‘We believe you have a radical Islamist in your midst.’ ” Nevertheless, Sen. Graham said, Mr. Tsarnaev was able to visit Dagestan and return without coming under suspicion, and discuss “killing Americans” openly on the Internet.

The bungling of warnings about the Boston Marathon bombers was followed this week by a failed effort to obtain more information from the surviving brother, Dzhokhar, who ceased answering investigators’ questions when he was formally read his “Miranda” rights to remain silent and have legal advice.

Fox News reported that Boston’s FBI office was “stunned” when federal District Court Judge Marianne Bowler read Dzhokhar Tsarnaev his rights in his hospital room. Under the Public Safety Exception to the Miranda rule, investigators have 48 hours to question suspects in cases where public safety may be at immediate risk. But questioning in this case was cut short at 16 hours, apparently with the support of the Justice Department’s lead prosecutor.

Sen. Graham had urged President Obama to treat Dzhokhar Tsarnaev as an unlawful combatant, a status that would have allowed thorough questioning before he was charged. That appeal was rejected by the White House.

The FBI acquitted itself extremely well in tracking down information after the bombings and in pursuing the suspects. But the task might not have been necessary if intelligence provided to the U.S. had been given adequate scrutiny. When another country alerts the U.S. to possible terrorists, the FBI should pay heed.