As investigations continue into the bombs shipped from Yemen to the United States, the news last week about a plot to bomb Washington, DC Metro stations should not be forgotten. Indeed, the arrest of Farooque Ahmed, a naturalized U.S. citizen born in Pakistan, is evidence of another kind of ongoing terrorist threat to the United States, one that stems from citizens and immigrants.

According to reports, Ahmed conspired to bomb four Metro stations in Arlington, Va., along with an unnamed DC hotel, with people he thought were al Qaeda operatives. They were, in fact, federal agents.

Ahmed, who became a naturalized citizen when he was 17 years old, entered the United States in 1993 and earned a bachelor’s degree from a New York college. He later relocated with his wife and young son to work in the D.C. area.

Ahmed’s arrest shows the threat of smaller-scale terrorist attacks carried out by U.S. citizens is growing. U.S. citizens are particularly attractive to al Qaeda and other terrorists because they can more easily plot and prepare for attacks without drawing the attention of law enforcement, the intelligence community or the Department of Homeland Security. Faisal Shahzad, the so-called Times Square bomber, was also a naturalized American from Pakistan. He first entered the United States on a student visa.

Ahmed and Shahzad share attributes: both planned attacks that did not include an aviation component (as many past terrorist attacks and attempted attacks have); both immigrated to the United States, fully assimilating to U.S. society, by most accounts; and at some point after their immigration, both became radicalized.

While the recent incident calls for an increase in ground transportation security, the event also demonstrates the continuing need to be extra vigilant in deciding who can enter the country. It is a challenging dilemma for immigration officials, particularly when it comes to foreigners who want to enter the country to study, as Shahzad did.

Bringing international students to study in the U.S. education system is an exceptional way of creating and maintaining intercultural relationships between the United States and other countries; however, it also opens the door to those who seek to abuse the system.

U.S. Immigration and Customs Enforcement stood up the Student and Exchange Visitor Program (SEVP) to track and monitor foreign students and exchange visitors throughout the duration of their time in the U.S. education system. SEVP collects and maintains foreign students’ information so that, ideally, only legitimate students can gain entry into and remain in the country. To carry out the mission effectively, SEVP works closely with the academic and law enforcement communities to close loopholes in the program while still maintaining the United States’ reputation as a welcoming country for foreign students and exchange visitors.

Sharing our educational system with top talents from across the globe provides invaluable benefits for students, their home country, and America as well, expanding cultural understanding and improving international relations. To maintain these opportunities, however, the United States must continue to build a strong security framework, such as with SEVP, to weed out those who mean America harm.