Police said a man, 27, from Birmingham, who was earlier in custody, and a 39-year-old woman who was on bail have been released with no further action.
Four people died and 50 were injured when Khalid Masood drove his car into pedestrians before stabbing a police officer. He was shot dead by police.
Fourteen people remain in hospital.
PC Keith Palmer was guarding Parliament when he was stabbed by Masood. He was unarmed, but the Metropolitan Police have confirmed that he was wearing a protective vest.
A post-mortem examination heard that there was no obvious evidence the vest had been damaged or penetrated, but it had been cut off when emergency first aid was given, the Met said.
It added that the coroner had requested that the provisional cause of death was not released at this stage.
Eleven people were initially arrested over the incident.
In a statement, the Met Police said the 39-year-old woman who was arrested in east London on suspicion of preparation of terrorist acts had been released.
Another women, aged 32, who was arrested in Manchester, remains on police bail while police continue their inquiries, it said.
The 58-year-old, who is still in custody, was arrested in Birmingham the morning after the attack under the Terrorism Act, meaning officers can hold him for up to two weeks.
Police are trying to establish whether Masood acted alone inspired by terrorist propaganda or if others supported him.
They are also looking into whether he used the encrypted messaging service WhatsApp to send a message two minutes before he struck.
Masood's other victims were Aysha Frade, who was in her 40s and worked at a London sixth-form college, US tourist Kurt Cochran, 54, and retired window cleaner Leslie Rhodes, from south London.
Janine Roebuck, a friend of 75-year-old Mr Rhodes, said he was "immensely proud" to have cleaned the windows at Chartwell - the Kent home of Sir Winston Churchill - for "many years".
"He was exceptionally fit for a man of his age. I would turn around and he had bounded up the ladder like someone half his age," she told BBC News.
"I am filled with huge sorrow that his life has been snatched from him in such an untimely manner."
The attack has brought into question security levels at Parliament, prompting a review by police and Parliamentary authorities.
Lord Blair, a former Met Police Commissioner who was in charge during the 2005 London bombings, said he was "absolutely certain that there will have to be changes" to the area near the entrance gates where PC Palmer was stabbed.
He refused to criticise officers for leaving the gates unguarded immediately after the attack, saying "at that moment they were behaving as human beings".
"That incident will have gripped them completely. They will know that the cavalry of the serious armed officers are on their way," he told BBC Radio 4's Today programme.
London attack: The victims
The path from violent crime to killer
The Metropolitan Police said Masood, 52, who had previous criminal convictions but none for terrorism, had used a number of aliases.
At birth, he was registered in Dartford, Kent, as Adrian Elms, but later took his stepfather's name becoming Adrian Ajao in childhood.
In the early 2000s, he was convicted of causing grievous bodily harm after slashing a man across the face with a knife in a pub.
Masood was believed to have been living in the West Midlands before Wednesday's attack, but had previously spent time in Luton, Crawley, Rye and Eastbourne.
The Saudi Arabian embassy said Masood had worked as an English teacher in the country for two years between 2005 and 2009.
In 2015, he obtained an Umra visa - allowing pilgrimage to Mecca - and was in the country from the 3 to 8 March.
The embassy in London said it "strongly" condemned the "terrorist act in London, as it does all forms of terrorism".
A 58-year-old man is still being held by police over the Westminster attack, as two others are released without charge.