A woman walks past a closed entrance of Russell Square underground station in London April 29, 2014. [Photo/Agencies]

LONDON - British police said on Thursday that mental health was a significant factor in a knife attack in central London that left one woman dead and five people injured but that they could not rule out terrorism as a motive.

Armed police arrested a 19-year-old man at Russell Square after he attacked a woman, who was in her 60s and who died from her injuries at the scene.

"Early indications suggest that mental health is a significant factor in this case and that is one major line of inquiry," London Metropolitan Police Assistant Commissioner Mark Rowley told reporters.

"But of course at this stage we should keep an open mind regarding motive and consequently terrorism as a motivation remains but one line of inquiry for us to explore."