NORTHFIELD, Minn. — EACH day, it seems there are new reasons to heed the growing threat of domestic terrorists. Just last Saturday, a radical Islamist group posted a video urging an attack on the Mall of America in Bloomington, Minn. In response, the secretary of Homeland Security, Jeh C. Johnson, advised visitors there to be “particularly careful.”

Concerns about the increasing frequency of recent terror attacks in Europe, Australia and North America caused the White House to convene a summit meeting last week on how best to counter violent extremism. But none of the current efforts to prevent homegrown terrorism in the United States will be sufficient without addressing the ease with which would-be terrorists in the United States can obtain firearms and explosives.

Protective measures include the tracking of travel patterns to and from certain countries, and tightened airport security, but our laws do nothing to stop domestic terrorist suspects from gaining access to the tools they need to inflict terrible damage. Those on the terror watch list are free to buy and own unlimited firearms in the United States.

And it is well documented that they do. The Government Accountability Office, the investigative arm of Congress, recently reported that between February 2004 and December 2014, individuals on the watch list attempted to purchase firearms or explosives on 2,233 occasions — and more than 90 percent of the time, they cleared a background check and received approval to buy.