Greenpeace activists' banner drop on Mount Rushmore National Memorial troubles members of South Dakota's congressional delegation.

Both Sen. Tim Johnson and Rep. Stephanie Herseth Sandlin said Friday that they want park officials to review security measures.

Herseth Sandlin said the memorial is faced with the difficult balance between providing security and allowing reasonable public access. She doesn't know if there were problems with the security system, but believes a review is important.

"I'm not comfortable that the monument is as secure as people want it to be," she said.

She also said the incident should spur an overall review of national monuments and other public places where people congregate and security is essential. That's important because a future attempt to avoid security could have a more nefarious intent, she said.

"When their assessment is complete, I may very well ask how this could have been prevented," she said. "They were aware of it. They responded. But under difference circumstances, if it wasn't Greenpeace unfurling a banner, is the system in place that worked as it was intended sufficient for other threats?"

Johnson said a thorough examination of the security system at the monument is needed.

"It's clear that system didn't work as intended and that they need to review procedures," said Johnson in a statement provided by staffers, a statement which seemed to contradict park superintendent Gerard Baker's pronouncement Thursday that 'all security measures functioned exactly as designed.'

"This time it was an indefensible, political stunt from a well-organized group who broke the law," read Johnson's statement, "but the National Park Service needs to examine what steps should be taken to protect our national treasure."

Sen. John Thune wasn't ready Friday to say the system malfunctioned, but he agreed that a thorough security review is needed.

"The extremists who perpetrated this week's publicity stunt and attempted to exploit our national treasure to promote their radical agenda should be fully prosecuted under the law," Thune said in a in a statement provided by his staff. "I am anxious to be briefed by officials from the National Park Service regarding their ongoing investigation and will work to make sure that any security gaps are addressed so that the monument is not exposed to future security breaches."