The mass shooting that left 14 people dead in San Bernardino on Wednesday has raised all manner of questions for law enforcement - especially, if and how it may have been prevented.

One of the alleged shooters, Syed Rizwan Farook, was a U.S. citizen born in Illinois. His wife and alleged accomplice, Tashfeen Malik, was originally from Pakistan; she arrived in the U.S. on a K-1 fiancee visa in July 2014, and was on her way to becoming a legal permanent resident.

Federal officials say they are now investigating the shooting as an act of terrorism. But neither of the two suspects had raised red flags with authorities in the past, including Malik, who was screened before entering the U.S. Could federal officials have done anything different?

KPCC spoke with Claude Arnold, until recently the agent in charge of U.S. Immigration and Customs Enforcement's Los Angeles office. Here are his thoughts on the case.

KPCC: When you look at this case, what kinds of questions come up for you?

Arnold: What it raises for me is concerns with the vulnerabilities of our immigration system. We only know about a person what we know. When someone is coming from another country, if we are not fortunate enough to have some sort of intelligence - through our own domestic intelligence or partnerships with other governments - that indicate they might be a threat, then we have no idea. It's not an exact science. We are always taking a chance when we allow someone to come into our country that we know nothing about. For the vast majority of people, it's not a concern. They would not be terrorists. But it only takes one or two to who have that intent to get through the system to potentially do us harm.

KPCC: What sort of scrutiny is given to people who enter the United States on a fiancee visa, as Tashfeen Malik did?

Arnold: They are subject to the same scrutiny as anyone who enters the U.S. on a non-immigrant visa. ICE, for example, based at various embassies abroad, they have visa security units that scrutinize applicants for non-immigrant visas to come to the U.S. And obviously, there is a specific emphasis on countries, people coming from the Middle East, who may be a threat. Someone entering on a fiancee visa would go through the same rigors as someone entering on a student visa, or a visitor's visa, et cetera.

KPCC: So if you are coming from a place like Pakistan, Saudi Arabia, Qatar, you would be subject to additional scrutiny?

Yes, absolutely. And again, they look at a variety of factors, not just the country they are traveling from, but they look at classified information, to see if that person is on anyone's radar screen, if there are any indications that the person has...had affiliations, had any connections to terrorism or terror suspects. And not just that. They are also reviewed just for basic fraud - is that person really coming as a fiancee in a bona fide relationship, or are they just using it as a means to enter the U.S.?

KPCC: Tashfeen Malik had recently received her conditional green card, which means she would have gone through even more screening, correct?

Arnold: Yes, that's correct. What that means, when someone gets their conditional resident status as the spouse of a U.S. citizen, that means...they have this two-year conditional residency period. The purpose of this is to make sure that it is a bona fide marriage, that someone just doesn't pay someone to marry them so they could get legal status. But even in that initial approval stage, again, U.S. Citizenship and Immigration Services would be running queries through various government databases, to make sure that there is no indication this person has any flags, that they might be involved in anything criminal, or with a national security concern.

KPCC: But she didn't raise any concerns. Local authorities say the same. The same goes for her husband, who was a U.S. citizen. Syed Farook traveled to Saudi Arabia, where they were married. Would this in itself have raised questions?

Arnold: In law enforcement, they don't rely on the travel histories alone. It's usually a series of things. And so it's not necessarily just travel to a single country. Is there any information regarding the purpose of that travel? Did they have a round-trip ticket so it looks like they are returning? What were the activities before they traveled? Someone who is traveling to engage in a conflict, they may undo a lot of their ties here in the U.S. before they travel. But if there is a lot of indication they intend to return to their normal life here, they don't quit their job. They don’t get out of their lease on their apartment, or sell belongings, et cetera. Then they are clearly intending to return back to their normal life. Now, that doesn't necessarily mean that they don’t intend to return to engage in some extremist activity here in the U.S. But there is a combination of things that are looked at, not just mere travel.

KPCC: So neither of them raised any red flags, even in the immigration process. What, then, to do?