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, the first version of which Mr. Trump announced a week into his presidency. The latest version of the ban, run by the State Department and Department of Homeland Security, had restricted travelers from eight countries — six of them predominantly Muslim — from entering the United States, though Chad was removed from the list last week.
A group that is challenging the ban — including several individuals, a Muslim organization and the State of Hawaii — argues that 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 past 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.