Two women were arrested in New York Thursday for allegedly plotting to conduct bombings with pressure cookers or propane tanks. The women, Noelle Velentzas, 28, and Asia Siddiqui, 31, were apparently inspired by the April 2013 Boston bombings, and had studied how to build explosive devices with the intent of attacking the U.S. homeland.

The investigation that led to the arrests was conducted by the Federal Bureau of Investigation’s Joint Terrorism Task Force and the New York City Police Department’s Intelligence Division. The Joint Terrorism Task Force is made up of local, state, and federal officials. Better coordination among local and federal officials and among the various intelligence-gathering agencies has contributed to the U.S. ability to disrupt and prevent nearly 60 terrorist plots in the U.S. since the 9/11 attacks.

This week’s arrests of the two women follow the apprehensions on March 25 of two men in Chicago on charges of conspiring to provide material support and resources to a foreign terrorist organization. In that case, Army National Guard Specialist Hasan Edmonds planned to travel to Syria to fight with the Islamic State of Iraq and al-Sham (ISIS), while his cousin Jonas Edmonds planned to attack the U.S. military installation at which Hasan had trained.

Both cases demonstrate that the U.S. must remain vigilant to homegrown terrorist threats as well as the foreign fighter phenomenon that is plaguing many European countries.

According to a recently released U.N. report, over 22,000 foreign fighters have joined ISIS in Iraq and Syria. The report calls the foreign fighters “an immediate and long-term threat” and the likelihood of a terrorist strike “growing, particularly with attacks targeting hotels, public spaces and venues.”

The number of foreign fighters throughout the world—including an additional 6,500 in Afghanistan and hundreds more in Yemen, Pakistan, Libya, and Somalia—has increased by 71 percent over the last year.

It is estimated that around 180 U.S. citizens, 600 British citizens, and 1,200 French citizens have travelled to Iraq and Syria to fight with ISIS. A total of around 3,400 of the foreign fighters come from Western nations.

The U.S. must keep up the fight against Islamist terrorists both at home and abroad. While an intensive drone campaign in Pakistan’s tribal border areas has degraded al-Qaeda’s core leadership, the group has adapted by empowering affiliate organizations in places like Somalia to conduct local attacks that destabilize governments.

The ISIS foreign fighter phenomenon has added an additional challenge to the U.S. fight against global terrorism that will require America to continue to remain on the offensive against the threat, partner closely with allies, and remain vigilant in protecting the U.S. homeland.