Imagine if even some of the 19 hijackers who boarded those planes on Sept. 11 had never been allowed into the US in the first place - or had been deported or arrested before the attacks.
Theoretically, if a high-tech visa system that's getting growing support in Washington had been in place - and functioning flawlessly - before Sept. 11, it could have stopped at least seven of the terrorists.
But that's a big "if." The idea is to track more thoroughly each entry and exit of the 300 million annual US visitors. It would use credit-card-like visas that include biometric measurements - probably fingerprints or hand prints.
President Bush budgeted $380 million for the idea. And a plan that has passed the House - now moving through the Senate - would lay the groundwork for the system.
But it faces tough realities. Observers say it's likely to be a useful - but hardly impregnable - shield in the war on terror.
"Can it keep track of everybody in a way that's comparable to implanting [electronic] chips in them?" asks Mark Krikorian of the Center for Immigration Studies. "No. But it would be a vast improvement over what we have now."
Many of the new measures being considered have actually been mandated by law since at least 1996 - but have been postponed or unfunded because of objections from business groups.
Clearly success of the new plan would rest on flawless execution of at least three key things:
First, careful tracking of each visitor's entry - and exit. Today, exits aren't monitored closely. So it's impossible to know if visitors remain in the US.
Under the new system, visitors would be given a visa card with a record, say, of their fingerprints digitally encoded on it. On arrival in the US, the card would be swiped through a reader to verify that fingerprints encoded in the card match the fingerprints in US databases. Visitors also might have to place their hands on a fingerprint reader.
To succeed, this system would also require the massive task of synchronizing numerous government databases.
Second, better enforcement is crucial. The INS has just 2,000 inspectors to chase millions of visa overstayers. Thousands more are needed to make a dent in the enforcement backlog, experts say. The new plan would boost funding for this - though it's not clear by how much.
Third, background screening is key. The system would rely heavily on frontline consular officers at US embassies who approve or deny visas. These officers are often pushed to process massive numbers of visa requests very quickly.
"There has to be a change in the culture and incentives - so they're actually given time to scrutinize applicants and are rewarded for being cautious and careful," says Jim Edwards, a lobbyist who backs the plan.
Even with full-blown success in these areas, the plan would be only a partial shield. It couldn't, for instance, ensure that a first-time visa applicant is actually who the applicant claims to be. Nor would it ensure that authorities would know where to find a person whose visa had expired.
But the system arguably could have stopped seven of the 19 Sept. 11 terrorists. Three of the men possessed expired visas. Under the new system authorities could have known they hadn't exited the US and might have tracked them down. Two men had student visas but hadn't shown up for classes. Authorities would have been notified of this under the new system and could have investigated. Two men were on an FBI watch list and could have been arrested. And one man apparently used someone else's passport to enter the country - near impossible with the biometric safeguard. Some of the hijackers could have been caught on more than one of these violations but, in all, the plans would have covered 7 of the 19.
Critics worry the new system is misguided.
"It would monitor millions in order to find a few dozen," says David Martin, former general counsel of the INS. The $380 million Bush is budgeting for it, he says, might buy better results through "old-fashioned law-enforcement."
The tech-heavy US approach means a tough road ahead for setting up the system. But the INS does have a schedule.
Existing law mandates that by October 2003, airlines and cruise ships must supply basic data to US authorities on each arriving and departing passenger. And they'll be fined $3,300 for each passenger missed. And by 2005, all border ports are supposed to collect this information.
Meanwhile, a Senate bill whose chief sponsors are Edward Kennedy (D) of Mass. and Dianne Feinstein (D) of California has strong support in the Senate. Among other things, it would require federal agencies to share immigration information. It would make all passports and visas tamper-resistant - including the new visa cards with biometric data. It would mandate that colleges notify the INS if their foreign students drop out or don't enroll.
Critics say the plan would choke US commerce - from Canadian and Mexican imports to the influx of foreign students. They suggest that keeping track of the half-billion US entries annually is impossible.
Yet as for the likelihood of it going forward, University of Maryland Prof. James Gimpel says, "There are very few politicians in Washington who can oppose this in the wake of 9/11."