WASHINGTON — Bombings in New York and New Jersey — and a stabbing attack in Minnesota the same day — underscore that homegrown attacks inspired by violent extremists are as much a threat to the United States as those directed by terrorists, the nation’s Homeland Security chief says.

While all attacks are difficult to detect and prevent, Homeland Security Secretary Jeh Johnson said the United States and its allies continue to “take the fight militarily to terrorist organizations overseas” 15 years after the Sept. 11, 2001, attacks on New York and Washington.

In prepared testimony before a Senate committee Tuesday, Johnson said air strikes and special operations against the so-called Islamic State terror group have led to the deaths of a number of its leaders. While it remains a threat, the Islamic State has lost nearly half the populated areas it once controlled in Iraq and thousands of square miles in Syria, Johnson said.

At the same time it loses territory, the group has “increased its plotting on targets outside of Iraq and Syria and continues to encourage attacks in the United States,” Johnson said.

Johnson, FBI Director James Comey and Nicholas Rasmussen, director of the National Counterterrorism Center, are set to testify Tuesday as the Senate Homeland Security and Government Affairs Committee looks at security threats 15 years after 9/11.

The panel’s chairman, Sen. Ron Johnson said the threat of “militant Islamic terrorist attacks to the United States remains significant,” citing the Sept. 17 attacks in the New York region and Minnesota, as well as deadly attacks in San Bernardino, Calif., and Orlando.

“In all, Islamic extremist terrorist have killed 63 people on U.S. soil since our committee last held its annual hearing to consider threats to the homeland,” Ron Johnson, R-Wis., said in a prepared statement.