(CNN) After President Donald Trump fired FBI Director James Comey, questions immediately arose about the President's motivations for his dismissal -- and for the recent firings of two other then-President Barack Obama-appointees who were in the middle of conducting investigations linked to Trump.

"They fired Sally Yates. They fired Preet Bharara. And they fired James Comey , the very man leading he investigation. This does not seem to be a coincidence," Schumer said shortly after the announcement, calling for a special independent prosecutor into the Trump campaign's ties to the Kremlin.

"Any person who he appoints to lead the Russian investigation will be concerned that he or she will meet the same fate as Director Comey," he said.

CNN's senior legal analyst Jeffrey Toobin was not buying the idea that Comey was sacked over the Clinton investigation, saying it was "absurd."

Toobin branded the move a "grotesque abuse of power by the President of the United States," comparing the sacking of Comey to President Richard Nixon's firing of special prosecutor Archibald Cox during the Watergate scandal.

James Comey

Then-FBI Director James Comey testifies in front of the Senate Judiciary Committee during an oversight hearing on Capitol Hill May 3, 2017 in Washington, DC.

The FBI director saw his reputation compromised when he became embroiled in the 2016 election campaign. He was first criticized by Republicans when he announced he wouldn't be charging then-Democratic nominee Hillary Clinton over her emails, and then by Democrats for publicly reopening the case days before Americans went to the polls.

Why was he fired?

The Trump administration attributed Comey's dismissal to his handling of the investigation into Democratic nominee Hillary Clinton's use of a private email server. In a signed letter released by the White House, Trump informed Comey that he was "hereby terminated and removed from office, effective immediately," explaining that he reached the conclusion that the erstwhile director was "not able to effectively lead the bureau."

What was he investigating?

As head of the FBI, he was overseeing the investigation into the Trump campaign's alleged ties to the Kremlin. Democrats have ridiculed the notion that the Clinton issue is what truly prompted Comey's dismissal, drawing parallels to Watergate-era firings and suggesting Comey was getting too close to the White House with the Russia probe.

Where is the investigation now?

At a hearing last week, Comey confirmed that the FBI's investigation into accusations of coordination between Trump's presidential campaign and Russian officials was continuing. It's not clear if the incoming FBI director will pick up where Comey left off.

Sally Yates

Former US Deputy Attorney General Sally Yates testifies before the Senate Judicary Committee's Subcommittee on Crime and Terrorism on Capitol Hill May 8.

Appointed by Obama, former Deputy Attorney General Yates had been running Trump's Justice Department as Acting Attorney General while Trump's nominee for the role, Sen. Jeff Sessions, awaited confirmation. She became a household name when Trump abruptly removed her from the temporary position.

Why was she fired?

Ostensibly for her refusal to implement the first iteration of Trump's ban on travelers from a number of Muslim-majority countries.

"The acting Attorney General, Sally Yates, has betrayed the Department of Justice by refusing to enforce a legal order designed to protect the citizens of the United States," White House press secretary Sean Spicer said in a statement at the time, explaining the President's actions.

What was she investigating?

As part of the probe into possible collusion between Russia and the Trump administration, then-acting Attorney General Yates met with White House counsel to inform them that then-National Security Advisor Michael Flynn wasn't telling the truth about his interactions with Russian ambassador Sergey Kislyak and, as a result, represented a blackmail risk.

"We believed that General Flynn was compromised with respect to the Russians," Yates said in a Senate subcommittee hearing aimed at gathering details of the Russian hacking of the 2016 election on Monday in Washington.

"Logic would tell you that you don't want the national security adviser to be in a position where the Russians have leverage over him," she added.

Where is the investigation now?

Yates said Monday that she warned the White House earlier this year that former Trump national security adviser Michael Flynn could be "essentially blackmailed by the Russians."

Preet Bharara

Preet Bharara, then-US Attorney for the Southern District of New York, at the US Attorney's Office for the Southern District of New York in June 2016.

Preet Bharara, former US Attorney for the Southern District of New York, was known as one of Wall Street's fiercest watchdogs and a widely respected prosecutor.

Why was he fired?

Bharara first refused to resign along with 46 US attorneys across the country. Although it is common for incoming administrations to replace US attorneys when transitioning to power, Trump had previously assured Bharara that he'd keep his job.

Sources told CNN that Bharara had been told after a meeting with Trump in November that he could stay on, and that he felt blindsided by the request. He was fired after refusing to comply.

At the time, Democratic Senator Elizabeth Warren posted a series of tweets suggesting Bharara was removed in part because he "had authority over Trump Tower."

Bharara suggested that this was indeed the case. "I wanted it to be on record that there was a deliberate decision to change (his) mind and fire me, particularly given what my office's jurisdiction is," he said.

What was he investigating?

Bharara's office had many investigations ongoing at the time of his firing, including one involving Trump favorite Fox News

And then there's the President's claim that he was wiretapped in Trump Tower on orders of then-President Obama, whose investigation led back to the Southern District of New York.

"Trump has undoubtedly decided that he wants his own pick rather than the choice of Senate adversary (and minority leader) Chuck Schumer in place as the top federal prosecutor in New York," CNN legal analyst Paul Callan wrote in March.

Where is the investigation now?