Six days after al-Qaida terrorists attacked New York, the Pentagon and my own home state, I was sworn in as the U.S. attorney for the Eastern District of Pennsylvania.

It was clear from my first day on the job that we were living in a world changed forever by these vicious attacks. Law enforcement and the intelligence community were charged with a new mission: to anticipate where and when acts of terrorism might be committed and to prevent them. It was a daunting responsibility.

Almost a full decade later, America remains on guard. Which is why, as a new Congressman, I am honored to have been asked to serve as chairman of the Homeland Security Subcommittee on Counterterrorism and Intelligence.

After such a short period of time with the subcommittee, I have a renewed understanding of the threat to our nation. I visited the National Counterterrorism Center and received a classified briefing from its director, Michael Leiter, on the terrorist threats against the United States.

I have met with members of the special operations community to discuss ongoing operations to hunt down members of al-Qaida. Three weeks ago, the committee heard from Homeland Security Secretary Janet Napolitano that the terrorist threat is “at its most heightened state” since the Sept. 11 attacks.

The threat is sobering and real. While al-Qaida is under pressure in Pakistan, the group has evolved in recent years into a more scattered network of affiliate organizations.

According to the Leiter, the most dangerous among these affiliates is al-Qaida in the Arabian Peninsula, most well known for the jetliner attack on Christmas 2009 and the more recent air cargo bombs. Radical Islamist cleric Anwar al-Awlaki — a U.S. citizen born in New Mexico — is one of that group’s top leaders, using his English-speaking ability to reach Westerners via the Internet and encouraging them to wage jihad.

Virtual recruitment of terrorists is a new front in the war against al-Qaida and illustrative of the enemy’s evolving nature. Our policies must evolve, too.

On Dec. 21, Attorney General Eric Holder said on “Good Morning America” that 126 individuals have been indicted on terrorism-related charges in the past two years, including 50 U.S. citizens. The new reality is that the enemy is among us — and it reared itself in the form of the car bomb in Times Square last spring and a gunman at Fort Hood, Texas, in November 2009, among others.

Law enforcement and intelligence officials cannot effectively combat the threat of terrorists without adequate tools to investigate suspects. That is why I voted to allow our intelligence community to use roving wiretaps, request documents and investigate “lone wolves” who act without the support of a terrorist organization. As a former federal prosecutor, I know firsthand how valuable these tools can be in terrorism cases. I also can attest to the multilayered protections in place to safeguard individual civil liberties, including review of warrants by federal judges. Congress recently voted to extend those authorities, which are part of the USA PATRIOT Act, but for less than three months. This was the latest in a series of continuing short-term reauthorizations. These provisions should be permanent but have been subject to a tug-of-war that puts politics before national security.

These investigative tools have been essential in preventing terrorist attacks. As the investigation of New York subway bomber Najibullah Zazi demonstrated, terrorists will seek to evade detection by using multiple e-mail accounts and cell phones. However, with the benefit of a warrant from Foreign Intelligence Surveillance Court, investigators were able to focus on the suspect Zazi. The ability to follow the target without the burden of seeking multiple warrants for separate e-mail and cell phone accounts, which would risk loss of critical intelligence in a fast-moving international terrorism case, enabled enforcement to act quickly and stop Zazi before he could strike. This is just one successful example of our law enforcement and intelligence officials using the tools provided by Congress to keep us safe.

Moving forward as subcommittee chairman, I will build on Chairman Peter King’s (R-N.Y.) counterterrorism priorities by examining the individual al-Qaida affiliate networks to identify the threat they each pose to the United States. The threat is more diverse, diffuse and less centralized. As such, it is now increasing and harder to detect, putting great strain on law enforcement and the intelligence community. Ten years after 9/11, it behooves us to examine the changing nature of the threat and adapt our policies accordingly. I intend to do so.

Rep. Patrick Meehan is one of 10 freshman Republicans on the House Homeland Security Committee. He was elected with 55 percent to succeed Democrat Joe Sestak, who ran for Senate, in Pennsylvania’s 7th district (suburban Philadelphia — most of Delaware County).