UK head of counter-terrorism Neil Basu says officers have disrupted far-right plots "designed to kill" and warned extremists were mimicking Islamist jihadist tactics.
David Parnham admitting sending the letters after he was arrested by counter-terrorism police.
She remains in police custody.
Martyn Hett's mother said she needed to understand "how someone turns into a monster".
UK police will be "stepping up" their presence at mosques as fears in the community have "significantly increased".
Since three packages containing explosive devices were found in three of London’s major transport hubs, workers have been warned to be extra vigilant for suspicious packages. Counter-terror police investigating the packages have circulated images of two of them to mail sorting offices and transport workers. No one was injured and no arrests have been made so far.
Counter-terror police have circulated images to mail sorting offices and transport workers
“The officers will deal with whatever they are confronted with.”
Neil Basu added that a no-deal EU exit would be "very bad" for policing.
Counter-terror detectives are scouring the home of the suspect.