In it, Cole Allen, 31, apologised to his parents, as well as colleagues and bystanders, for what he was about to do.
Follow live - shooting at correspondents' dinner attended by Trump
He said: "I apologise to everyone... who suffered before I was able to attempt this, to all who may still suffer after, regardless of my success or failure."
He said he may have given "a lot of people a surprise today", and although he did not name Donald Trump directly in the writings, he did criticise him and mentioned targeting the administration.
Explaining why he was going to do what he intended to do, he said "what my representatives do reflects on me", adding that "this is the first real opportunity I've had to do something about it".
Read more:
What we know about suspected gunman
The threats against Trump's life over the years
How shooting unfolded at White House correspondents' dinner
In the message, he said administration officials were targets, as were Secret Service agents "only if necessary, and to be incapacitated non-lethally if possible".
He said hotel guests and employees were "not targets at all" and that Capitol police, National Guard and hotel security were not targets "unless they shoot at me".
He said: "In order to minimise casualties, I will also be using buckshot rather than slugs (less penetration through walls)", but added he "would still go through most everyone here to get to the targets if it were absolutely necessary".
He also said he did not "expect forgiveness", before criticising the security at the hotel, and signing off with the name Cole "coldForce" "Friendly Federal Assassin" Allen.
The Metropolitan Police said he was arrested at an address near Barnstaple, Devon, on Sunday on suspicion of preparing terrorist acts. He has been taken to a London police station for questioning.
There have been a total of 26 arrests over a series of attacks on Jewish sites in north west London since an attack on Jewish community ambulances in Golders Green in March.
There have also been arson attacks on a synagogue and the former offices of a Jewish charity.
Another incident saw a drone flown near the Israeli embassy and premises linked to people opposing the Iranian regime have also been targeted, the Met said last week.
An attempted arson attack was carried out earlier this month outside the offices of a Persian media company.
Counter terror police have been investigating the possible involvement of Iranian proxies.
Eight people have been charged with arson-related offences and one person has been convicted of arson.
A 17-year-old boy last week pleaded guilty to arson not endangering life after an attack on Kenton United Synagogue.
Speaking at the synagogue on Thursday, Sir Keir Starmer said: "We have to deal with malign state actors, that will require legislation. We're going to bring that legislation forward.
"It's really important we do that. It's very important this particular context because I'm increasingly concerned that a number of countries are using proxies for attacks in this country."
Read more:
Man arrested over fire at LGBT+ nightclub
Car bomb explodes next to police station
At a roundtable discussion with leaders of the Jewish community, including the UK's Chief Rabbi Sir Ephraim Mirvis, Sir Keir said the fight they face to practice their religion freely is "all of our fight".
"This is not a government that is coming to support you in your fight," he continued. "It's a government that says this is our fight as well because this is the Britain that we believe in - that tolerant, reasonable Britain."
She performed in the 1960s group, best known for hits including Be My Baby, Walking in the Rain, Baby, I Love You and (The Best Part of) Breakin' Up, alongside her cousins Ronnie Spector and Estelle Bennett.
The band's official Instagram page shared the news of her death, calling her "a light" to those who knew her.
The post, which showed Talley Ross sitting in front of a portrait of the band, said: "It is with heavy hearts that we share the news of Nedra Talley Ross' passing. She was a light to those who knew and loved her.
"As a founding member of The Ronettes, along with her beloved cousins Ronnie and Estelle, Nedra's voice, style and spirit helped define a sound that would change music.
"Her contribution to the group's story and their defining influence will live forever.
"Rest peacefully dear Nedra. Thanks for the magic."
Talley Ross' daughter, Nedra K Ross, also shared the news of her mother's death on Facebook.
She wrote: "At approximately 8:30 this morning our mother Nedra Talley Ross went home to be with the Lord.
"She was safe in her own bed at home with her family close, knowing she was loved. Thank you Lord. There will be a Celebration of Life in the future and I will post information when plans have been confirmed."
The group, which formed in 1957, were known for their signature beehive hair and rocketed to fame due to their collaboration with music producer Phil Spector.
The only released one studio album - Presenting the Fabulous Ronettes Featuring Veronica (1964) - but their music has stood the test of time.
The Ronettes were inducted into the Rock and Roll Hall of Fame in 2007.
General Sadio Camara's death was confirmed in a post by the defence ministry on social media, while state TV said a car loaded with explosives was driven into his home in the town of Kati, about 9 miles (15km) north of Mali's capital, Bamako.
Over the weekend, regional al Qaeda affiliate, Jama'at Nusrat al-Islam wal-Muslimin, known as JNIM, along with the Azawad Liberation Front or FLA, a Tuareg-dominated rebel group, said they had carried out simultaneous attacks in more than half a dozen areas on Saturday, including the capital.
The separatists have been fighting for years to create an independent state in northern Mali, while al Qaeda and Islamic State-aligned militants have been battling the government for more than a decade.
A spokesperson for the FLA said the Russian Africa Corps troops and the Malian military withdrew from the city of Kidal following an attack on Saturday, after an agreement was reached for their peaceful exit.
Saturday's attacks were the first time the separatists joined forces with JNIM.
"This operation is being carried out in partnership with the JNIM, which is also committed to defending the people against the military regime in Bamako," said FLA spokesperson Mohamed El Maouloud Ramadan.
The separatists called on Russia to "reconsider its support for the military junta" in Mali, saying its "actions have contributed to the suffering of the civilian population".
Kidal was previously a stronghold of the rebellion before being taken by Malian government forces and Russian mercenaries in 2023. Its capture marked a significant, symbolic victory for the junta and its Russian allies.
While the exact fate of Kidal remains uncertain, General Oumar Diarra, head of the armed forces, confirmed on state TV that the Malian army had left the city and that troops were repositioning in Anefis, a city about 62 miles (100km) south of Kidal.
Saturday's strikes also hit near Bamako airport and further north, including Mopti, Sevare and Gao.
The government has said the assaults, which it described as terror attacks, appeared to be over.
Previously, it said civilian and military personnel were among 16 people wounded, but did not provide a figure for how many had died. It said several militants were killed.
Read more from Sky News:
Cole Allen: Trump gun suspect
Netanyahu's rivals merge
Chernobyl 40 years on
The Economic Community of West African States condemned Saturday's attacks in Mali and called for a "co-ordinated effort to combat this scourge".
The United Nations delivered a similar plea, adding that the secretary-general was "deeply concerned" and "strongly condemns these acts of violence".
The ex-prime ministers, right-wing Naftali Bennett and centrist Yair Lapid, issued statements on Sunday announcing the merger of their parties, Bennett 2026 and There is a Future.
Lapid, standing alongside Bennett at a joint news conference, said: "We are standing here together for the sake of our children. The State of Israel must change direction."
Bennett said the new party would be called Together, and that he will be its leader.
Read more:
Buckingham Palace confirm King's US state visit to go ahead
Israeli army destroys solar panels in Christian Lebanese town
He said: "After 30 years it is time to part with Netanyahu and open a new chapter for Israel."
Their bid will commence in the upcoming election expected later this year, they added.
The duo have joined forces before, when they put an end to Netanyahu's successive 12-year tenure in a 2021 election.
But they formed a coalition with a thin majority that survived just 18 months.
Netanyahu, Israel's longest-serving prime minister, made a comeback when he won the November 2022 election.
A year later, the Middle East was plunged into turmoil following Hamas's 2023 attack on southern Israel.
Israel retaliated immediately and carried out airstrikes on Gaza.
In mid-December, Gaza's Hamas-run health ministry said the death toll in the territory had passed 45,000.
Since then, polls have predicted he will lose the next election due by the end of October.
Bennett, 54, an ex-army commando turned tech millionaire, has been trailing Netanyahu in election polls.
A survey on 23 April by Israel's N12 News found Bennett securing 21 of the 120 seats, against 25 for Netanyahu's Likud.
It found Lapid's party securing only seven seats, down from the 24 it currently holds.