President Donald Trump attacked the FBI and lawmakers probing suspected Russian meddling in the 2016 U.S presidential election, drawing sharp criticism for saying an excessive focus on Russia led investigators to miss signs that could have prevented a deadly school shooting in Florida.

In a series of tweets over the weekend from his Mar-a-Lago estate in Florida, Trump said congressional investigations and political "hatred" showed Russia had succeeded in sowing "discord, disruption and chaos" in the United States.

He accused his Democratic predecessor, Barack Obama, of failing to do enough to stop Russian election interference.

"They are laughing their asses off in Moscow," Trump tweeted on Sunday morning.

In a tweet on Saturday night, Trump criticized the Federal Bureau of Investigation for missing warning signs in the case of Nikolas Cruz, 19, who is charged with killing 17 people on Wednesday at Marjory Stoneman Douglas High School in Parkland, a Fort Lauderdale suburb.

The FBI acknowledged on Friday that it failed to investigate a warning that Cruz possessed a gun and the desire to kill.

"Very sad that the FBI missed all of the many signals sent out by the Florida school shooter. This is not acceptable," Trump wrote. "They are spending too much time trying to prove Russian collusion with the Trump campaign - there is no collusion," he added.

The Republican president offered no evidence of any link between the investigation of Russian meddling and the FBI's failure to prevent the Florida shooting, the deadliest-ever at a U.S. high school.

Student survivors of the massacre slammed Trump for connecting it to the Russia probe, which has shadowed his year-old presidency.

"Oh my god. 17 OF MY CLASSMATES AND FRIENDS ARE GONE AND YOU HAVE THE AUDACITY TO MAKE THIS ABOUT RUSSIA???!!" Morgan Williams, 16, tweeted. "HAVE A DAMN HEART."

Several lawmakers, including a fellow Republican, also rejected Trump's linking of the FBI's missteps in preventing the shooting to the Russia probe.

"So many folks in the FBI are doing all that they can to keep us safe. The reality of it is, is that they are two separate issues," Republican Senator Tim Scott said on CBS' "Face the Nation" programme.

Former Deputy Attorney General Sally Yates tweeted: "Our president uses the tragedy to attack the investigation of a foreign adversary's interference in our democracy. Shameful."

Yates, who had been a holdover from the Obama administration, was fired early in the Trump presidency for refusing to defend travel restrictions on several Muslim-majority countries.

The FBI did not immediately respond to a request for comment on Trump's tweet.

INDICTMENTS

On Friday, Special Counsel Robert Mueller charged 13 Russians and three Russian companies with conspiracy to tamper in the 2016 U.S. election.

Mueller's indictment said the Russians adopted false online personas to push divisive messages and staged political rallies while posing as Americans.

In one of 13 tweets Trump sent after Mueller's indictment, the president said he "never said Russia did not meddle in the election."

Trump has on several occasions, however, questioned whether Russia was behind efforts to interfere in the 2016 election.

U.S. spy agencies concluded more than a year ago that Russia used hacking and propaganda to try to tilt the election in favour of Trump. Russian President Vladimir Putin has repeatedly denied that.

After Mueller's indictment was made public on Friday, Trump said it backed up his assertion that there was no collusion between his campaign and Russia.

The indictment did not address whether anyone from the Trump campaign coordinated or worked with Russians. Mueller's broader probe is ongoing.

"It wasn't designed to be able to clear everything on the investigation," Republican Senator James Lankford said on NBC's "Meet the Press."

"It was designed to say it was very clear these 13 individuals in this set of companies were trying to interfere in our election," said Lankford, who sits on the Senate Intelligence Committee.

Trump, in a Sunday morning tweet, belittled Representative Adam Schiff, the senior Democrat on the House Intelligence Committee that is investigating Russia's actions, labelling him a leaking "monster."

Schiff, speaking on CNN's "State of the Union" programme, said details of the indictment provided "overwhelming and unequivocal" evidence of the threat from Russian interference and countered Trump's assertions that the Russia investigation was a "hoax."

"This is a president who claims vindication anytime someone sneezes," Schiff said.