MSNBC host Andrea Mitchell fretted Friday that the FBI's acknowledged error in not following up on a credible trip on Nikolas Cruz might serve as fodder for critics of the Russia investigation.

In a statement, the FBI said Friday that "protocols were not followed" after someone contacted the bureau's Public Access Line on Jan. 5 with concerns about Cruz, who authorities say confessed to carrying out the Florida school shooting this week. The individual provided information in January about Cruz's desire to kill people, erratic behavior, disturbing social media posts, and potential to carry out a school shooting.

The FBI said under established protocols, the caller's information was sufficient cause to assess Cruz as a threat to life and should have been forwarded to the Miami field office for further investigation, but this did not occur.

Former FBI executive assistant director Shawn Henry said Friday the FBI's conduct was "unacceptable."

"There's absolutely no excuse," Henry said on MSNBC's "Andrea Mitchell Reports."

"Somebody dropped the ball. It's inappropriate, it's unacceptable, and I don't know what to say," Henry added. "I've been on the air before and, when appropriate, defended the FBI with many of their actions. In this case, it's clear that somebody along the line, based on the statement of Director [Christopher] Wray, missed something that led to this tragedy or could have potentially disrupted this."

Mitchell said this news came at a time when the FBI is "very vulnerable" because of the attacks from President Donald Trump and the White House over its handling of the Russia election interference investigation.

"It's completely unrelated to what they're doing on the Russia probe, but this could feed the public animus that has already been ginned up by critics of the Russia investigation, including the president," Mitchell said.

"There's no doubt that this will bring critics out, and let me be very, very clear. What happened here is completely unacceptable," Henry said.

Henry said the error didn't take away from the FBI's thousands of successes.

The 19-year-old Cruz confessed to the murders of 17 people at Marjory Stoneman Douglas High School in Parkland, Fla., according to a police report. It is the third-deadliest school shooting in American history.