LONDON — Britain on Monday identified two of the three suspects in the weekend’s terrorist assault in London, including one who not only was well-known to the police but had a cameo in a television documentary on homegrown violent jihadists.

The identifications immediately suggested that the police and intelligence services had missed a series of red flags in failing to stop the attack, which left seven people dead and dozens wounded in a nation still grappling with the Manchester concert bombing two weeks earlier.

The terrorist attack on Saturday was the third to strike Britain in three months, all with the participation of radicalized British Muslims who had been on the radar of law enforcement but still plotted successfully.

The frequency of the attacks has created the impression of a sudden campaign against Britain hatched from within. But counterterrorism officials and others say the country has been a terrorist target for years. Even now, there are hundreds of investigations underway.