Tamerlan Tsarnaev, a U.S. citizen and the son of Muslim immigrants, was investigated by the FBI for connections to Islamic radicals. They dropped the investigation. He and his brother killed three people and injured 264 others at the Boston Marathon in April 2013.

Syed Farook, a U.S. citizen and the son of Muslim immigrants, was investigated by the FBI for connections to Islamic radicals. They dropped the investigation. He and his wife, herself a Muslim immigrant, killed 14 and injured 22 others in San Bernardino in December 2015.

Omar Mateen, a U.S. citizen and the son of Muslim immigrants, was investigated by the FBI twice for connections to Islamic radicals. Twice they dropped the investigation. He killed 49 and injured at least 50 others in Orlando in June 2016.

One such incident is a black eye on the FBI. Two is a pattern. Three is an outrage that calls for a congressional investigation.

But don’t hold your breath waiting for one. That would require at least one member of Congress to be willing to endure the inevitable comparisons with Sen. Joseph McCarthy.

Let’s be clear: The FBI has investigated many similar cases involving U.S. citizens, professing Muslims, who are either recent immigrants, the children of immigrants, or converts to Islam. They have arrested and prosecuted a number of suspects, and many of those cases have never made national news.

I’ve written extensively for Chronicles about one such case here in Rockford, Illinois (which was related to at least two others); I spent hours interviewing the FBI’s confidential informant who broke the case and who has worked on other such cases around the country. The problem runs far deeper and wider than most people would like to admit.

Those arrests and prosecutions are the success stories, and those who wish to defend the FBI will no doubt point to them. But if you are tempted to declare that the current system is working because there have been only three obvious incidents in which the FBI clearly failed, you should ask yourself this: What would failure look like?