Washington (CNN) -- More U.S. Marines are fortifying security at the American Embassy in Nairobi following the deadly shopping mall attack there, CNN has learned.
Two U.S. officials confirmed the move in Kenya, but declined to be identified because they are not authorized to speak publicly about the matter.
The newly arrived Marines are part of a unit specially trained in enhanced security, including providing personal protection to senior American officials.
That kind of assistance was recently added to tasks performed by the embassy guard program after the deadly terror attack on the U.S. diplomatic compound in Benghazi, Libya, last year.
Ambassador Christopher Stevens and three other Americans were killed in the armed assault that, in part, focused attention on State Department security readiness and military response.
The security step also follows a State Department travel advisory on Wednesday about potential terror threats in East Africa from al Qaeda and affiliated groups following the Westgate Mall attack and standoff that killed 67 people.
Due to security concerns, the officials would not discuss the precise number of Marines added to the embassy since the shopping mall attack.
While there is no specific threat against the embassy at this time, the Obama administration deemed it a "high threat" facility, the officials said.
The United States has doubled the number of Marine guards at the Nairobi embassy in the past eight weeks.
The force was first increased during a heightened state of alert during which the United States close nearly two dozen embassies worldwide due to a threat from al Qaeda in Yemen. Security was beefed up again prior to the anniversary of the 9/11 attacks.
Separately on Thursday, Attorney General Eric Holder said there was no evidence so far to indicate that any Americans were involved in the Nairobi attack.
Citing a U.S. official with direct knowledge of the situation and the latest security assessment, CNN reported on Wednesday that authorities want to know if any Americans were among the attackers as claimed by the Somali-based al-Shabaab terror group said to be behind it.
Kenyan authorities have said the attackers were from a number of countries, but have not confirmed that any Americans were involved.
"We don't have verifiable information that would indicate one way or the other way whether any Americans were involved," Holder said. "That is one of the things that we are trying to do with our Kenyan colleagues - go through the forensic things, materials that we have, physical things to make that kind of determination."
United States knows so far that the attack that began on Saturday was carefully planned.
FBI experts are assisting the Kenyans on a preliminary basis. They have specific expertise in analyzing explosives and fingerprints.
"FBI agents have been on the ground since right after the incident began. We have been flowing resources into Nairobi on a continuing basis. We have had conversations about this matter both here and at the White House," Holder said.
American forensic investigators were granted access to mall alongside Kenyan officials on Thursday, CNN's Evan Perez reported.
But U.S. law enforcement officials told Perez that there is an increasing concern that some of the perpetrators escaped alongside fleeing victims, making the effort to piece together who was responsible more difficult.
The updated State Department travel caution on security threats and terror activities worldwide included the Kenya attack, saying al-Shabaab had "taken credit" for it.
"Al-Shabaab assassinations, suicide bombings, hostage taking, and indiscriminate attacks in civilian-populated areas are also frequent in Somalia," the advisory said.
Holder added that there was no "specific, credible evidence" that al-Shabaab is planning anything domestically.
"I am not sure they have the capacity to do anything in the United States," he said, adding that the United States is taking the "threat that they pose" seriously.
A U.S. official with access to the latest intelligence told CNN the State Department advisory was issued out of an "abundance of caution."
But officials are concerned there could be other plots in the works.