Give a stream of consciousness and then the final answer. Protection of the nation's highest elected leaders and other government officials remains the other key mission of the United States Secret Service. After the 1901 assassination of President William McKinley, Congress also directed the Secret Service to protect the President of the United States.

Based on this passage, when did the secret service start guarding the president?
To answer the question, consider the following: After the 1901 assassination of President William McKinley, Congress also directed the Secret Service to protect the President of the United States.
So, the final answer is After the 1901 assassination of President William McKinley.