Updated

The State Department announced Friday it has been quietly offering a reward of up to $10 million for information leading to the arrest and conviction of anyone involved in the deadly 2012 terrorist attack in Benghazi.

Friday’s acknowledgement comes weeks after Fox News reported that the Benghazi case was not listed as part of the State Department's "Rewards for Justice" website. The apparent omission prompted several complaints from Republican lawmakers.

But the State Department, in a letter sent to Rep. Michael McCaul, R-Texas, revealed that it has in fact been offering the reward through other channels since January -- though the department previously declined to disclose that information.

Friday’s announcement of a reward raised questions among some lawmakers about how effective a multi-million dollar reward could be if it was not widely known or publicized on its “Rewards for Justice” website.

However, in the letter to lawmakers, the State Department defended its decision to keep the reward under wraps.

"Due to security issues and sensitivities surrounding the investigation, the event-specific reward offer has not been publicly advertised on the RFJ website," the department said in a statement. "RFJ tools can be utilized in a variety of ways, without publicizing them on the website."

A State Department official familiar with the letter sent to McCaul by Assistant Secretary of State for Legislative Affairs Julia Frifield acknowledged that it's unusual not to publicize offers of rewards, but said investigators have other ways of making sure the information is known "as needed."

In the course of the probe, investigators have made it known to individuals that cash is available for those coming forward with actionable information.

The official said the rewards have been in place since Jan. 7, while Hillary Clinton was still secretary of state.  The official was not authorized to speak publicly about the private correspondence and spoke on condition of anonymity.

Lawmakers had complained the department was not using everything at its disposal to catch the perpetrators.

McCaul, chairman of the House Homeland Security committee, had been the lead author of an Oct. 30 letter to Secretary of State John Kerry asking why rewards were not being offered for the Benghazi attackers. Eighty-two other lawmakers signed that letter.

The State Department had previously ducked questions about whether rewards for the Benghazi attackers had been offered, citing concerns about identifying possible suspects.

The refusal to discuss the issue had led to criticism from many, mostly conservative, lawmakers who believe that the administration has badly mishandled Benghazi and may have even attempted to cover up key details about the attack that occurred on the 11th anniversary of the 9/11 terrorist attacks in the United States.

Earlier this month, Sen. Ted Cruz, R-Texas, introduced a bill to offer a $5 million reward for information on the Benghazi terror attack or information leading to the capture of a suspect.

“The State Department’s Rewards for Justice Program exists to help the U.S. identify and apprehend its enemies, but the Obama Administration has not used it to pursue the terrorists who attacked our personnel in Benghazi,” Cruz said in a written statement.

The program has distributed over $125 million to more than 80 people since 1984, Cruz said.

“This legislation enables the Secretary of State to offer a substantial reward for information leading to the apprehension and prosecution of the suspects who have been identified,” Cruz said. “U.S. investigators should have all available tools at their disposal to bring to justice those who murdered four Americans in Benghazi, including the first Ambassador killed in service since 1979.”

Since the attack, there have been five House panels and an internal State Department investigation looking into the incident. The attackers have still not been caught.

Republicans have raised concern that the administration still has not been forthcoming.

Fox News is told that the House Intelligence Committee is looking into why, for instance, CIA personnel were asked to sign a non-disclosure agreement after the attack.

Fox News has obtained a copy of the document signed by at least five CIA personnel. While it does not make reference to Benghazi, it was the second such document they were asked to sign, which is unusual. And it makes clear that disclosures could mean severe consequences, including criminal prosecution.

Fox News' Catherine Herridge and The Associated Press contributed to this report.