The online message referred to the Labour MP using a racial slur and said he would “hang from a lamppost” if he “wasn’t careful”.
But two months later, the MP said the force had dropped their investigation because the social media giant had “refused to assist”.
He tweeted on Wednesday evening: “I have just been informed by @metpoliceuk they have dropped their investigation into this threat because @Twitter has refused to assist in their investigation.”
Tagging Twitter founder Jack Dorsey, he asked: “Please explain why you are shielding vile racists who make death threats on your platform? #BlackLivesMatter.”
Mr Lammy later added: “You should not be able to push race hate and send death threats with impunity online.
“Shame on Twitter for failing to act with the Metropolitan Police to identify who sent this threat.
“Black Lives Matter has to be more than a slogan to drive traffic and ad revenue on your website.”
A Twitter spokesperson said: “We are cooperating with the Metropolitan Police, having now received and processed the correct information.”
The force has been contacted for comment.
When the racist tweet was first sent, it took several days for the account to be suspended and the messages removed.Politicians, including Home Secretary Priti Patel and Labour leader Sir Keir Starmer, condemned the post and urged social media platforms to act more quickly against hate.Sir Keir said: “The racism David Lammy, black MPs and black people across our society are subjected to is disgraceful.