(CNN) President Donald Trump lashed out at the FBI late Saturday, describing its failure to follow up on a tip about Florida school shooter Nikolas Cruz as "very sad."

The shooting reignited demands for tougher gun laws, with student survivors gathering in Fort Lauderdale on Saturday to criticize Trump and other lawmakers for their inaction.

But Trump shifted blame to the FBI, saying it is devoting too much time to investigating his presidential campaign.

"Very sad that the FBI missed all of the many signals sent out by the Florida school shooter," the President tweeted a day after he visited hospitalized survivors of the shooting.

"This is not acceptable. They are spending too much time trying to prove Russian collusion with the Trump campaign -- there is no collusion. Get back to the basics and make us all proud!"

Very sad that the FBI missed all of the many signals sent out by the Florida school shooter. This is not acceptable. They are spending too much time trying to prove Russian collusion with the Trump campaign - there is no collusion. Get back to the basics and make us all proud! — Donald J. Trump (@realDonaldTrump) February 18, 2018

Trump maintains denials

On Friday, the Justice Department announced that special counsel Robert Mueller has issued indictments on 13 Russian national s and three Russian entities for allegedly meddling in the 2016 presidential election, charging them with conspiracy to defraud the United States.

Photos: Deadly Florida school shooting Photos: Deadly Florida school shooting Parents wait for news after a shooting at Marjory Stoneman Douglas High School in Parkland, Florida, on Wednesday, February 14. At least 17 people were killed at the school, Broward County Sheriff Scott Israel said. The suspect, 19-year-old former student Nikolas Cruz, is in custody, the sheriff said. The sheriff said Cruz had been expelled for unspecified disciplinary reasons. Hide Caption 1 of 13 Photos: Deadly Florida school shooting Police officers ride in the back of a pickup truck as they tend to a victim. Hide Caption 2 of 13 Photos: Deadly Florida school shooting People are brought out of the Marjory Stoneman Douglas High School after the shooting. Hide Caption 3 of 13 Photos: Deadly Florida school shooting Police officers surveil the exterior of the school while the shooting was active. Hide Caption 4 of 13 Photos: Deadly Florida school shooting Students and faculty are evacuated from the school. Hide Caption 5 of 13 Photos: Deadly Florida school shooting A father and daughter embrace after a mass shooting at the Marjory Stoneman Douglas High School. Hide Caption 6 of 13 Photos: Deadly Florida school shooting Family members wait for word from students beneath an underpass just south of the campus of Marjory Stoneman Douglas High School. Hide Caption 7 of 13 Photos: Deadly Florida school shooting Lavinia Zapata embraces her son, Jorge, after he was evacuated from the school. Hide Caption 8 of 13 Photos: Deadly Florida school shooting The suspect, 19-year-old former student Nikolas Cruz, is taken into custody by authorities. Cruz was apprehended off of the campus. Hide Caption 9 of 13 Photos: Deadly Florida school shooting Security instructs parents following the shooting. Parents were told they would be reunited with their children at a local Marriott Hotel. Hide Caption 10 of 13 Photos: Deadly Florida school shooting Students released from lockdown console each other. While some students were evacuated, many remained in the school until authorities could come to their aid. Hide Caption 11 of 13 Photos: Deadly Florida school shooting Family members embrace after shooting. Hide Caption 12 of 13 Photos: Deadly Florida school shooting People embrace while leaving the Fort Lauderdale Marriott Coral Springs Hotel, which authorities designated as a staging point for witnesses to the shooting. Hide Caption 13 of 13

Mueller convened a grand jury as part of his investigation into Russian interference in the 2016 election as well as any possible connections between Russia and Trump campaign associates.

The sweeping indictment describes an unprecedented campaign by Russia to influence the election, affirming the longstanding conclusions of the US intelligence community.

It is at odds with Trump's repeated questioning of those conclusions.

Trump has said that despite the charges, the Russian meddling had no effect on the outcome of the election. In a separate tweet late Saturday, he said "the only collusion was between" Russia and Hillary Clinton.

'Focus on what's real here'

Asked by CNN's Fredricka Whitfield about the President's tweet, Nicole Hockley, whose six-year-old child was murdered in the Sandy Hook Elementary School shooting, said she thought Trump was trying to link the FBI's actions on Russia and the Florida shooting in an effort to distract.

"Just ignore all that," Hockley said. "You can't conflate these two things. They have nothing to do with each other. Was the Parkland shooting preventable? Absolutely. Does it have anything to do with Russia? No."

She advised people to "focus on what's real here" and not be thrown off of the shooting with the President's insertion about the Russia investigation.

FBI's admission

The FBI has acknowledged receiving two tips that appear to relate to Cruz ahead of the shooting. One was a January 5 call to a tip line from someone close to him -- one that the FBI said it failed to act on

The caller provided information about "Cruz's gun ownership, desire to kill people, erratic behavior, and disturbing social media posts, as well as the potential of him conducting a school shooting," the bureau said.

The information should have been assessed as a "potential threat to life," but the proper protocols weren't followed and the FBI's Miami office was not notified, the agency said.

FBI Director Christopher Wray said the bureau is investigating what happened.

"We have spoken with victims and families, and deeply regret the additional pain this causes all those affected by this horrific tragedy," Wray said in a statement.

The FBI's admission prompted Florida Gov. Rick Scott to call on Wray to resign.

In addition to the call, a video blogger said he warned the FBI in September about a possible school shooting threat from a YouTube user with the same name as Cruz.

An FBI agent confirmed a field officer in Jackson, Mississippi, received the tip and interviewed the person who shared it.

But no additional information was found to help identify the person who posted the comment and no connection was made to South Florida, said FBI special agent Robert Lasky, who is in charge of the Miami division.

The FBI's missteps have raised questions about whether it could have prevented the shooting. Attorney General Jeff Sessions ordered a review Friday into how the Justice Department and FBI respond to indications of potential violence.

"We will make this a top priority," Sessions said in a statement. "It has never been more important to encourage every person in every community to spot the warning signs and alert law enforcement. Do not assume someone else will step up -- all of us must be vigilant. Our children's lives depend on it."

Sen. Marco Rubio, R-Florida, also called for the House and Senate to conduct investigations into how the FBI reviews public tips in similar instances.