Twitter left hundreds of Russian propaganda videos, with millions of views, on its video platform Vine for months after it should have known they were posted by a Kremlin-linked troll group.
The discovery raises new questions about the nature of the company's effort to find and remove content produced by Russians trying to meddle in American politics, and how comprehensive it has been.
The accounts and videos were removed only after CNN brought them to Twitter's attention on Wednesday. Twitter did not comment as to why it removed the accounts or why they had been allowed to remain live for so long.
Sen. Mark Warner, the leading Democrat on the Senate Intelligence Committee, told CNN he was concerned by the findings and Twitter's response.
"Twitter shouldn't wait for Congress or anybody else to send a 'to-do' list with specific accounts to delete," Warner said in a statement given to CNN. "The company needs to take responsibility and be proactive about stopping Russians and other bad actors who are abusing its platform."
Last fall, Twitter provided Congress with a list of 2,752 accounts that it said were linked to the Internet Research Agency, a troll group based in St Petersburg, Russia, with ties to the Kremlin.
Related: The Kremlin-linked troll Twitter can't seem to shake
Among the accounts Twitter gave Congress were @GUNS4LIFE_ME and @PoliceStateMe. Both Twitter accounts were suspended, but their associated Vine accounts, @GUNS4LIFE and @PoliceState, were still live as of Wednesday morning.
The Vine accounts were first discovered by a Twitter user, who alerted CNN to their existence.
Vines are six second videos that are looped on repeat. Vine was acquired by Twitter in 2012, but in 2016 Twitter announced it was effectively closing the service by disabling future video uploads. However, Twitter maintains the Vine archive -- meaning videos posted to the site before uploads were disabled can still be viewed and shared.
The Police State Vine account posted more than 600 videos showing incidents of alleged police misconduct in the United States. The account began posting in September 2015 and stopped suddenly in August 2016. 25,000 accounts subscribed to the account's feed, and its videos were "looped" more than 6 million times.
The Police State Vine account links back to the now-suspended Police State Twitter account. CNN found evidence through cached pages that the Twitter account shared content from the Vine account, showing the accounts were associated.
Another account, GUNS4LIFE, posted more than 600 videos that were looped more than 8 million times. That account also appeared to be active from September 2015 before stopping suddenly in August 2016.
Through archived tweets, CNN found the suspended GUNS4LIFE_ME Twitter account linked to the associated Vine account dozens of times. The full archive of the account's tweets might have contained more links to the Vine videos.
Asked about Russian troll activity on Vine on Wednesday, Twitter told CNN, "We have suspended all known Vine accounts we were able to connect with Twitter accounts previously suspended and linked to the IRA."
Shortly after, when CNN showed Twitter the two still-active Vine accounts, the accounts were suspended. Twitter provided no further comment about them.
Twitter also declined to say whether it would hand the details of suspended Vine accounts over to Congress. Instead it pointed to a company blog post from January that said, "When we appeared before the United States Congress last fall, Twitter publicly committed to regularly updating both congressional committees and the public on findings from our ongoing review into events surrounding the 2016 U.S. election."
