Visas used by foreigners to travel to the United States are getting new scrutiny in the wake of shooting massacres in California and France.

House lawmakers passed legislation Tuesday tightening controls on visa-free travel and requiring visas for anyone who has been in Iraq or Syria in the previous five years.

Some lawmakers said they also planned to re-examine a visa that Tashfeen Malik used to come to the country. She is the Pakistani woman who helped her American husband carry out the San Bernardino attack that killed 14 people.

Here's a breakdown of how some of those visas work:

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WHAT IS THE VISA WAIVER PROGRAM?

It allows citizens of 38 countries to travel to the United States without a visa for stays of 90 days or less.

Travelers must submit data through an electronic counterterrorism screening program maintained by the Department of Homeland Security before boarding a plane, but they are not required to show up at a local consulate to apply for a visa as tourists from other countries must do.

Those who fail to pass the screening are required to apply for a visa.

About 20 million visitors come to the United States each year for business or tourism under the program, largely from European countries.

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WHAT CHANGES HAVE BEEN PROPOSED TO THE PROGRAM?

House lawmakers passed legislation to deny visa-free travel to anyone who has been in Iraq or Syria in the last five years.

The move aims to prevent attacks in the U.S. by citizens of countries that participate in the visa waiver program. The concern is that most of the Paris attackers were citizens of Belgium and France.

The bill proposed by Republicans and backed by the White House also would require participating countries to share counterterrorism information with the United States.

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WHAT IS A FIANCE VISA AND HOW IS IT DIFFERENT?

A fiance visa, also known as K-1, is issued abroad to people who plan to marry American citizens upon traveling to the United States.

It is one of the smallest visa categories managed by the U.S. — accounting for 0.3 percent of the nearly 10 million visas issued in fiscal year 2014 — and was created to try to root out marriage fraud.

Applicants must give at least one in-person interview, fingerprints, be checked against U.S. terrorist watch lists and have their family members and travel and work histories reviewed.

Malik entered the U.S. in July 2014 on a K-1 visa and married American citizen Syed Farook a month later. She was approved by the U.S. government as a conditional resident and was eligible to seek a permanent green card in two more years.

Since the San Bernardino shootings, the Obama administration has said it is reviewing the program.

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HOW DOES REFUGEE SCREENING COMPARE WITH THE VISA PROCESS?

Lawmakers also are looking into the screening process for refugees fleeing Syria's civil war.

Some Republican lawmakers have raised concerns that terrorists might try to hide among those genuinely seeking safe haven.

Like K-1 visa applicants, refugees must submit to in-person interviews overseas and provide their fingerprints and biographical information to U.S. officials.

However, the process takes much longer, and U.S. government officials say foreigners applying from Iraq and Syria are given additional scrutiny.

Those awarded refugee status can apply for permanent green cards after a year.