Tuesday's deadly, coordinated attacks on the Brussels airport and Metro line marked the third major terrorist attack to have rattled the Europe Union since the beginning of last year.
Each was carried out by terrorists sharing the radical Islamist ideology pushed by the Islamic State. And they may include some overlapping conspirators. But we now know that the attacks all featured brothers who had spent time in prisons.
Most of the estimated 30 deaths in Tuesday's bombings in Brussels, Belgium were caused by suicide bombings by brothers Brahim and Khalid el-Bakraoui.
Both had extensive criminal histories and spent time in prison. In 2010, Ibrahim was sentenced to prison for armed robbery. The next year, Khalid received a prison sentence for carjacking and possession of a weapon.
As I have written extensively, prisons can serve as incubators for radical Islamists.
The Brussels attack came days after authorities finally tracked down Salah Abdeslam, a key conspirator and the only known surviving terrorist from November's multi-targeted attacks in Paris which killed as many as 130 people and wounded more than 350.
Abdeslam's brother, Ibrahim, blew himself up in the attack at the Voltaire Cafe. Salah had extensive criminal history for petty crimes and was jailed in 2010 for robbery.
That was the second terrorist attack to strike Paris in 2015. In January, brothers Cherif and Said Kouachi entered the offices of the Charlie Hebdo magazine and killed 12 people. The attackers said they were avenging Islam's prophet Muhammad, who had been depicted in several unflattering images by the satirical magazine.
A subsequent attack at a Jewish deli by an accomplice of the Hebdo shooters resulted in the deaths of five more people, including a French police officer, Clarissa Jean-Philippe.
The Kouachis were known to authorities for prior criminality. Cherif spent almost two years in the Fleury-Merogis prison where he met and was radicalized by a fellow inmate and al-Qaida associate named Djamel Beghal. Cherif Kouachi's cellmate was Amedy Coulibaly.
Coulibaly was responsible for killing the French police officer and the four hostages in the Jewish deli.
Common threads, prior criminality, prison radicalization, brotherhood.
The United States is not immune from these links. The 2013 Boston Marathon bombing was carried out by brothers Tamerlan and Dzhokhar Tsarnaev.
In 2010, the New York State Police did a study on known terrorists and issued its findings in the Vigilance Report, a comprehensive analytical report to identify trends in major U.S. terrorism cases since 9/11. It showed a nexus to prior criminal history, be it arrest, probation, imprisonment, or parole, in nearly half of the perpetrators.
A more recent concern is the findings of the Congressional Research Service, which reports that within the next five years, more than 100 inmates in prison for terror-related crimes will be released. In light of that development, Assistant Attorney General for National Security John Carlin recently spoke of the dim prospect of these individuals having been cured of their radicalization tendencies. "There are [rehabilitation] programs for drug addicts and gang members [in prison]. There is not one [program] with a proven track record of success for terrorism," he said.
There is a registry for sex offenders when released, yet we have none for known terrorists. This omission doesn't seem right.
So where does that leave us? With a recidivism rate between 40-65 percent of common criminals, what is the prospect of one of these soon-to-be released terrorists returning to the jihad?
If we do not seriously address the problem beforehand we may find ourselves facing the same types of attacks as our European NATO allies in the not-too-distant future.
Patrick Dunleavy is the former Deputy Inspector General for New York State Department of Corrections and author of The Fertile Soil of Jihad. He currently teaches a class on terrorism for the United States Military Special Operations School.