WASHINGTON — President Trump took office promising to enact an “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 report released on Tuesday by the Cato Institute concluded that the United States already had a robust vetting system that had 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 of 531 people convicted of or killed in carrying out terrorism-related crimes since Sept. 11 had entered the United States despite the security screening improvements. That compares with 52 people 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 one for every 379 million visa or status approvals from 2002 through 2016.