Presidential candidate Herman Cain will receive protection from the United States Secret Service, a source in federal law enforcement and two other sources told CNN.

Cain will be the first candidate in the race for the Republican presidential nomination in the 2012 election cycle to be placed under the protection of this federal law enforcement agency.

It is not yet clear why Cain is getting Secret Service protection.

While early, it is not unprecedented for the Secret Service to take over the security of a presidential candidate. In May 2007, then-Sen. Barack Obama, who was running for the Democratic nomination, was placed under Secret Service protection. It was the earliest a candidate for president had received protection from this law enforcement agency. At the time, another Democratic candidate, then-Sen. Hillary Clinton, was already under Secret Service protection because she was a former first lady.

Usually, the Secret Service begins protecting “major” candidates designated by an advisory committee, which consists of the congressional leadership, four months before the general election, according to guidelines outlined on the agency’s website.

(Source: CNN)

The Secret Service began protecting the president after President William McKinley was assassinated in the fall of 1901. The agency extended its protection to major presidential candidates following the assassination of Robert Kennedy in 1968.