Sen. Dianne Feinstein, D-Calif.

Since it was exposed in June by leaker Edward Snowden, the National Security Agency’s call-records program has become controversial and many have questioned whether its benefits are worth the costs. My answer: The program — which collects phone numbers and the duration and times of calls, but not the content of any conversations, names or locations — is necessary and must be preserved if we are to prevent terrorist attacks.

In the summer of 2001, the CIA’s then-director, George Tenet, painted a dire picture for members of the Senate Intelligence Committee when he testified about the terrorist threat posed by al Qaeda. As Mr. Tenet later told the 9/11 Commission, “the system was blinking red” and by late July of that year, it could not “get any worse.”

A similar level of concern emerged in July 2013, when the Senate Intelligence Committee was briefed on the al Qaeda threat that led the State Department to close an unprecedented 19 American diplomatic facilities across Africa and the Middle East.

Our safety depends on the ability of the 16 U.S. intelligence agencies—including those at State, in the military, and at the FBI—to discover unfolding plots by tracking connections between terrorists, especially plots tied to the U.S. homeland. This is why NSA’s call-records program is an essential component of U.S. counterterrorism efforts.

Consider the case of 9/11 hijacker Khalid al-Mihdhar, who was being watched by the CIA while he was in Malaysia. U.S. intelligence agencies failed to connect the dots before the attack to recognize that al-Mihdhar had flown with (future) hijacker Nawaf al-Hazmi to Los Angeles in January 2000.

Intelligence officials knew about an al Qaeda safe house in Yemen with ties to al-Mihdhar as well as the safe house’s telephone number, but they had no way of knowing if anyone inside the U.S. was in contact with that phone number in Yemen. Only after 9/11 did we learn that al-Mihdhar, while living in San Diego, had called the safe house.

In congressional testimony in June, FBI Director Bob Mueller said that if intelligence officials had had the NSA’s searchable database of U.S. telephone-call records before 9/11, they would have been able to connect the number to al-Mihdhar and produce actionable intelligence on participants of the developing plot. NSA Director Keith Alexander testified before Congress in October that if the call-records program had existed before 9/11, there is a “very high” likelihood that we would have detected the impending attack that killed 3,000 Americans.

Working in combination, the call-records database and other NSA programs have aided efforts by U.S. intelligence agencies to disrupt terrorism in the U.S. approximately a dozen times in recent years, according to the NSA. This summer, the agency disclosed that 54 terrorist events have been interrupted—including plots stopped and arrests made for support to terrorism.  Thirteen events were in the U.S. homeland and nine involved U.S. persons or facilities overseas. Twenty-five were in Europe, five in Africa and 11 in Asia.

These figures show that the NSA programs are a key component of our counterterrorism efforts at home and abroad because they develop intelligence for our allies about terrorists operating within their borders.

The U.S. must remain vigilant against terrorist attacks against the homeland. Al Qaeda in the Arabian Peninsula (AQAP), considered the world’s most capable and dangerous terrorist organization, is determined to attack the United States. As we have seen since the “underwear bomber” attempted to blow up an airliner over Detroit on Christmas Day
2009, AQAP has developed nonmetallic bombs that can elude airport screeners, and the organization’s expert bomb maker, Ibrahim al-Asiri, remains at large.

Asiri is believed to be behind the October 2010 plot to place bombs disguised as printer cartridges onto cargo planes headed for the U.S. He is also a suspect in the May 2012 suicide-bomber plot against an airliner headed for the U.S. that was foiled when U.S. authorities obtained the planned explosive device through good intelligence work.

Earlier this month, Director of National Intelligence James Clapper testified that in the case of the AQAP threat this summer, there were a number of phone numbers or emails “that emerged from our collection overseas that pointed to the United States.” Fortunately, the NSA call-records program was used to check those leads and determined that there was no domestic aspect to the plotting.

The NSA call-records program is working and contributing to our safety. It is legal and it is subject to strict oversight and thorough judicial review.

I believe we should increase the program’s transparency and its privacy protections. Toward that end, the Senate Intelligence Committee will soon consider a bill to make improvements to these counterterrorism programs. The proposed legislation will, for example, require court review when the call records are queried, and mandate a series of limitations on how the records can be obtained, stored and used.

But we must also learn the lesson of 9/11. If we end this vital program, we only make our nation more vulnerable to another devastating terrorist attack.

U.S. Sen. Dianne Feinstein, D-California, is chairman of the Senate Intelligence Committee. Her commentary originally appeared in the Wall Street Journal.