Mrs Webber also said the families were told the inquiry would be concluded within two years, and Dr Kumar thanked the prime minister "from a father to a father".
The families of Barnaby Webber, Grace O'Malley-Kumar and Ian Coates were told at an emotionally charged meeting at Number 10 that a "number of different agencies" would be scrutinised by the probe.
Sir Keir Starmer has told the families of those killed in the Nottingham attacks that a judge-led public inquiry will start in "a matter of weeks".
Students Mr Webber and Ms O'Malley-Kumar, both 19, and 65-year-old caretaker Mr Coates were killed by Valdo Calocane before he attempted to kill three other people in a spate of attacks in the city in June 2023.
Dr Sanjoy Kumar added: "As we have always said as families, everywhere that Valdo Calocane intersected with the authorities, we were let down.".
