WASHINGTON — President Trump took office promising to enact a so-called “extreme vetting” immigration system with a travel ban that he said would keep terrorists from exploiting the visa system and getting into the United States.
But a new report released on Tuesday by the Cato Institute concluded that the United States already had a robust vetting system in place that has largely been successful because of a series of intelligence and security reforms implemented after the Sept. 11, 2001, attacks.
The report found that just 13 out of 531 individuals convicted or killed in carrying out terrorism-related crimes since Sept. 11 had entered the United States despite the security screening improvements. That compares to 52 individuals in the 15 years before the 2001 homeland security reforms, the report found.
Only one of those 13 participated in a deadly attack in the United States, the report found — a rate of or one for every 379 million visa or status approvals from 2002 through 2016.
“The evidence indicates that the U.S. vetting system is already ‘extreme’ enough to handle the challenge of foreign terrorist infiltration,” said David J. Bier, author of the report and an immigration policy analyst at the Cato Institute, a libertarian think tank in Washington.
The report was based on an examination of court records and documents from the Department of Homeland Security and the State Department. Mr. Bier said it also included data from a list of unsealed international terrorism and terrorism-related convictions from the Justice Department.
The Cato Institute report comes a week before the Supreme Court hears arguments in a challenge to the travel ban that Mr. Trump announced a week into his presidency. The ban, run by the State Department and Department of Homeland Security, restricts travelers from eight countries — six of them predominately Muslim — from entering the United States.
A group that is challenging the ban — including several individuals, a Muslim organization and the state of Hawaii — argues the order amounts to a broad exclusion of visitors based largely on religion and national origin.
The Justice Department defended the executive order as barring a large number of people “about whom the United States lacks sufficient information to assess the risks they pose.”
Mr. Bier said the government created a number of new agencies and improved methods to identify suspicious travelers after the 2001 attacks. That included creating terrorist watch lists, purchasing technologies to collect digital fingerprints and facial scans, hiring thousands of new border customs and immigration officers and revamping the visa approval process.
The Obama administration also tightened the American visa waiver program, making it harder for travelers to enter the United States from Europe if they had dual citizenship from Iran, Iraq, Sudan or Syria, or had visited one of those countries in the last five years. The administration later added Libya, Somalia, and Yemen to the restricted list.
The Cato report concluded that the gamut of security measures, which reaches across the globe, already reduces the risk of terrorists slipping into the United States through the immigration system to attack.
“While people of all types — foreign-born or U.S.-born — will always pose certain risks to the country, the country has maxed out its capacity to improve immigration vetting,” Mr. Bier wrote.