Al-Qaeda’s attacks on Sept. 11, 2001, had a profound impact on Americans who felt vulnerable and doubted their government’s ability to protect them.  Even though there have been security improvements since 9/11 ,serious problems persist.

Consider six post-9/11 security improvements and the challenges that must be overcome to keep America safe.

First, public transportation has improved security especially with airlines but problems persist.

After 9/11, the Transportation Security Administration​ (TSA) was established to handle airline passenger pre-screening and flight security.  The TSA quickly increased security by hardening cockpits, banning items like knives and liquids greater than three ounces, and toughening passenger screening.

Anyone who has flown since 9/11 is aware of the sometimes groping indignities even the aged must endure as a result of tighter passenger screening.  But even these draconian measures suffer serious shortfalls.  Specifically, explosives detection technology lacks reliability, body scanning machines are ineffective at detecting explosives and the TSA should do more profiling.

We also need to improve our watch-listing capability with information-sharing between intelligence and immigration authorities.  Several attempted attacks should have been detected by the U.S. immigration system, such as the known al-Qaeda Christmas Day bomber Umar Farouk Abdulmutallab​, who used a valid U.S. visa to board Northwest Flight 253 in Amsterdam.

Second, our anti-terrorism war has badly damaged the core al-Qaeda, but the organization’s fight will now come from its franchises and grassroot jihadists.

Al-Qaeda’s new leader, Ayman al-Zawahiri, threatens to conduct an attack more terrible than 9/11.  Zawahiri just released an Internet message:  “Seek to attack America that has killed the imam of the mujahideen [Osama bin Laden] and threw his corpse in the sea and then imprisoned his women and children.”

But Zawahiri lacks the operational capability to launch another 9/11-style attack because of our past battlefield successes and his recent loss.  Last week a CIA drone attack killed Zawahiri’s operations officer, Atiyah Abd al-Rahman.  Rahman was in charge of coordinating attacks against the U.S. and Europe, and managing its far-flung affiliates.  But al-Qaeda’s regional franchises and grassroot jihadist affiliates are quite capable.

The Yemen-based al-Qaeda in the Arabian Peninsula (AQAP), which controls much of Yemen, and its Somali partner, Al-Shabaab, have shown both the means and intent to conduct transnational attacks.  In October 2010, AQAP placed explosives onto U.S.-bound flights.  Last week, al-Qaeda in the Islamic Maghreb claimed responsibility for a suicide attack in Algeria and on the same day Boko Haram, a Nigerian terrorist group aligned with Al-Shabaab, killed dozens of people when it blew up the United Nations headquarters in Nigeria’s capital, Abuja.

But the major threat to the American homeland will come from homegrown operatives who are more difficult to identify.  Last year, 10 Muslim-Americans plotted against domestic targets and five carried out their plots.  Somali communities in Minneapolis and Portland, Ore., are ripe for Al-Shabaab recruiters and others are “self-radicalized” via the Internet.

Third, we need better border security and immigration reform because foreign terrorists exploit these weaknesses.

Eighteen of the 19 9/11 hijackers obtained 30 state-issued identifications that enabled them to easily board planes.  That is why the 9/11 Commission recommended federal standards for the issuance government sources of identification such as driver’s licenses.

The federal government passed the REAL ID Act in 2008 to establish an identification standard.  But to date, only one-third of the states have complied with that law, which creates vulnerabilities and makes us less safe.

Besides, we have a serious illegal immigrant problem for other two reasons.  Our government has operational control of less than half of the porous 2,000-mile Southwest border.  And our visa overstays account for almost half of the illegal immigrants in our country.

We need to control the entire border and enforce the visa system through our biometric entry-exit screening system.  This system checks all individuals who arrive at U.S. borders, checks their identities, and helps prevent known terrorists from entering the country.

Immigration authorities also need to give employers the tools needed to make sure the person they are hiring is legally here.  The current system lacks a verifiable real-time electronic network that, once in place, would quickly identify illegal immigrants.

Fourth, former President George W. Bush said, “The biggest threat facing this country is weapons of mass destruction in the hands of a terrorist network.”  Bush knew al-Qaeda tried to acquire nuclear weapons in the late 1990s and would have used one in America given the opportunity.

There are nearly 2,000 tons of highly enriched uranium in the world, enough to make thousands of nuclear weapons.  Keeping that material out of the hands of terrorists, “who would surely use them,” was the reason President Obama gave for hosting the 2010 Nuclear Security Summit.

Unfortunately, the 49 nations represented at the summit are not the problem.  Rogue regimes such as North Korea and our erstwhile ally Pakistan, both nuclear powers, are known weapon proliferators.  More work is needed to prevent these and other countries like Iran from proliferating fissile material and securing known stockpiles.

Fifth, federal law enforcement has overcome many impediments, especially the timely sharing of terrorism-related information but much work remains to be done.

Since 9/11, the Federal Bureau of Investigation​ (FBI) shifted significant resources to international counterterrorism and intelligence gathering with the help of the Director of National Intelligence (DNI), which oversees the intelligence community.  Those changes have enabled the bureau to disrupt many terrorist plots.

But problems remain for federal law enforcement regarding the terrorism threat, such as the failure to foil the shooting in Fort Hood, Tex.  The FBI​ knew the shooter had become radicalized under the influence of an al-Qaeda extremist, but nothing more than a superficial inquiry resulted.

Finally, America’s foreign policy is a two-edged sword.  Our support of “Arab Spring” revolutions sweeping across the Middle East may help remove tyrants, but the ensuing instability will likely be exploited by terrorists, as appears to be the case in Libya.

The U.S. should align itself with emerging pro-democracy governments that are inhospitable to terrorists.  But our ongoing wars in Iraq and Afghanistan, according to Pew Research Center surveys, have alienated most Muslims.

That is why most Muslims say they don’t trust America’s support for Arab Spring revolutionaries.

Libya illustrates the challenge.  Although the U.S. supported the Libyan rebels’ victory over dictator Muammar Gaddafi​, the end result may be something other than pro-West.  For example, Abdel-Haim al-Hasadi was named commander of the Tripoli military council.  He was once the leader of the Libyan Islamist Fighting Group, an al-Qaeda affiliate.  Other known Islamists are among the Libyan rebels and may eventually rule that country to our disadvantage.

The 9/11 terrorists had a profoundly dramatic impact on America.  Our subsequent security initiatives kept the country safe, but they must be improved upon if America is to be secure against a morphing and sophisticated enemy.