WASHINGTON (Reuters) - The raid by FBI agents on Monday on the offices and home of U.S. President Donald Trump’s longtime attorney Michael Cohen grew out of an investigation by Special Counsel Robert Mueller that has cast a cloud over Trump’s presidency.

FILE PHOTO: Special Counsel Robert Mueller departs after briefing members of the U.S. Senate on his investigation into potential collusion between Russia and the Trump campaign on Capitol Hill in Washington, U.S., June 21, 2017. REUTERS/Joshua Roberts/File Photo

Mueller is investigating whether the Trump campaign and Russia colluded to help Trump win the 2016 U.S. presidential election and whether Trump has unlawfully sought to obstruct the probe. Trump has called the investigation a “witch hunt.”

Following is a timeline of major events surrounding the investigation.

June 2015 - Donald Trump, a wealthy real estate developer and reality TV personality, announces his candidacy for the Republican presidential nomination.

March 2016 - Around this date, Russia’s military intelligence agency, the GRU, begins a cyber campaign aimed at interfering with the 2016 presidential election, according to U.S. intelligence agencies.

April 2016 - Trump foreign policy aide George Papadopoulos meets with an academic who has just returned from Moscow. The academic tells Papadopoulos that the Russians have obtained dirt on Democratic presidential candidate Hillary Clinton, including “thousands of emails.”

June 9, 2016 - Trump’s son Donald Trump Jr., Trump’s son-in-law Jared Kushner and campaign manager Paul Manafort meet at Trump Tower in New York with a Russian lawyer and others. Emails later made public show Trump Jr. believed he would receive information harmful to Clinton.

July 5, 2016 - Former British spy Christopher Steele, who was investigating Trump’s Russia connections in an effort paid for by the Democratic National Committee (DNC), briefs an FBI agent on his findings.

July 22, 2016 - On the eve of the Democratic presidential nominating convention at which Clinton became her party’s nominee, WikiLeaks releases 44,000 emails hacked from the DNC. The content of some of the emails prompts the resignation of DNC chairwoman Debbie Wasserman-Schultz.

Late July 2016 - The FBI begins a counter-intelligence investigation of Russian meddling in the election.

Aug. 19, 2016 - Manafort resigns as Trump’s campaign chairman following news reports of his business dealings in Ukraine.

Sept. 5, 2016 - U.S. President Barack Obama meets Russian President Vladimir Putin during a G-20 summit in China and warns him of a strong response if Russia’s meddling continues.

Oct. 7, 2016 - Within an hour of the airing of an “Access Hollywood” video in which Trump talks in vulgar terms about women, WikiLeaks begins serial publication of thousands of private emails belonging to Clinton campaign chairman John Podesta.

The U.S. Department of Homeland Security and Office of Director of National Intelligence issue a statement that for the first time publicly blames Russia for the election-related computer hacks.

Nov. 8, 2016 - Trumps wins the U.S. presidential election.

Dec. 29, 2016 - Obama, in response to the hacking and harassment of U.S. diplomats in Moscow, places sanctions on Russian intelligence agencies and individuals, expels 35 Russian diplomats and shuts Russian-owned compounds in Maryland and New York.

After the sanctions are announced, Trump national security aide Michael Flynn holds a series of phone calls with Sergei Kislyak, the Russian ambassador to the United States.

Jan. 6, 2017 - In an unclassified report, the U.S. intelligence community states that Putin ordered an election meddling effort whose goals eventually included helping Trump and harming Clinton.

President-elect Trump is briefed by U.S. intelligence chiefs on the finding, and is told of the existence of information gathered by Steele.

Jan. 10, 2017 - BuzzFeed publishes the Steele “dossier” detailing alleged collusion between Trump’s campaign and Moscow and containing salacious allegations regarding Trump.

Jan. 20, 2017 - Trump is sworn in as president.

Feb. 13, 2017 - Flynn resigns as Trump’s national security adviser, reportedly having misled Vice President Mike Pence and others about his talks with Kislyak.

March 20, 2017 - FBI Director James Comey for the first time publicly confirms the bureau’s Russia counter-intelligence investigation.

May 9, 2017 - Trump fires Comey, and days later attributes the dismissal to “this Russia thing.”

May 17, 2017 - Deputy Attorney General Rod Rosenstein, the No. 2 Justice Department official, appoints former FBI director Robert Mueller as special counsel to investigate possible collusion between the Trump campaign and Russia.

July 26, 2017 - Federal agents execute a pre-dawn raid of Manafort’s home.

Oct. 30, 2017 - As part of Mueller’s investigation, Manafort and business partner Rick Gates are indicted on money-laundering and other charges. Manafort pleads not guilty. Gates later pleads guilty to lesser charges and cooperates with Mueller’s probe.

Papadopoulos pleads guilty to lying to the FBI about his Russia contacts and agrees to cooperate with the special counsel.

Dec. 1, 2017 - Flynn pleads guilty to lying to the FBI and agrees to cooperate with the special counsel.

Feb. 16, 2018 - Mueller charges 13 Russian individuals and three Russian companies, including the St. Petersburg-based Internet Research Agency, with conspiracy to tamper with the 2016 election.

April 3, 2018 - Alex van der Zwaan, the Dutch son-in-law of one of Russia’s richest men, is sentenced to 30 days in prison and fined $20,000 for lying to Mueller’s investigators, becoming the first person sentenced in the special counsel’s probe.

April 9, 2018 - FBI agents raid the offices and home of Trump’s personal attorney, Michael Cohen. The raid is in part related to Cohen’s payment of $130,000 to porn star Stormy Daniels to keep quiet about an affair she said she had with Trump.