Story Highlights• NEW: Durbin says he brought "racially motivated" information to Reid's attention
• Homeland security secretary authorizes protection detail
• No specific, credible threat known, sources say
• Hillary Clinton already protected as former first lady
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(CNN) -- Democratic presidential candidate Barack Obama, a U.S. senator from Illinois, has been placed under the protection of the Secret Service, the agency said Thursday.
The government is not aware of any specific, credible threat against Obama, according to a law enforcement source familiar with the decision. But his office has received hate mail and calls and other "threatening materials" in the past and during his campaign, the source said.
Three Obama campaign officials who discussed the issue on condition of anonymity also said there was no specific threat against the candidate.
They said the request stemmed from what one called the "cumulative effect" of a heavier campaign schedule, larger crowds and "just the growing perception internally" it was time to take additional security precautions that are best suited for the Secret Service.
A Department of Homeland Security spokesperson also told CNN there is no known specific or credible threat.
Illinois' senior senator, Democrat Dick Durbin, told reporters Thursday night that he relayed concerns about the size of the crowds Obama was drawing and other issues to Senate Majority Leader Harry Reid.
Reid decided to take the matter to Homeland Security Secretary Michael Chertoff as an issue for a congressional advisory board, Durbin said.
"I knew the crowds were large ... but some of the other information given to us, unfortunately I think, raised a concern among many of [Obama's] friends," Durbin said.
"Unfortunately, some of the information we found was racially motivated. It is a sad reality in this day and age that Mr. Obama's African-American heritage is a cause for very violent and hatred, hated reactions among some people."
Durbin would not elaborate. "I've been advised not to talk about any specific security problems or any threats," he said. He also would not say how he received the information, only that it was from "credible sources."
The crowds, he said, have been record-breaking. "Naturally, it's encouraging politically, but it's also raised a lot of security concerns."
The Secret Service protection for Obama began at 1 p.m. Thursday, Durbin said.
Chertoff works with a congressional panel made up of half a dozen members of Congress, including House Speaker Nancy Pelosi and Reid.
The decision to present the information to the advisory board was a bipartisan one, Durbin said, with Senate Minority Leader Mitch McConnell, a Kentucky Republican, also participating, he said.
Previously, two Democratic Senate sources told CNN that after Reid decided to take the matter to Chertoff as an advisory board issue, further discussions with the Obama campaign ensued and the official request for Secret Service protection was made.
The Secret Service said in a written statement that Chertoff, "after consultation with the congressional advisory committee, authorized the United States Secret Service to protect presidential candidate Senator Barack Obama."
Protection goes beyond surrounding the candidate with well-armed agents, the Secret Service's Web site states. The agency does extensive advance work and threat assessments developed by its intelligence division to identify potential risks, the site says.
"As a matter of procedure, we will not release any details of the deliberations of assessments that led to protection being initiated," the Secret Service statement said.
Another Democratic presidential candidate, New York Sen. Hillary Clinton, already has Secret Service protection because of her husband's presidency.
Other candidates could request Secret Service protection. The Secret Service is authorized to provide protection to "major" candidates as determined by the advisory committee, under certain guidelines.
Among those guidelines, the candidate must be announced, be actively campaigning in at least 10 states and have some degree of prominence in the polls.
Durbin said he hopes any candidate who feels there is a threat against them will "go through the same process and ask for protection."
CNN's Jeanne Meserve contributed to this report.
Sen. Barack Obama, at left, in white shirt, shakes hands in a crowd of supporters last week in South Carolina.
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