An 18-year-old man has been arrested in the brutal beating death of an 86-year-old woman, and is suspected of attacking a 92-year-old on the day of the murder.

Neven Glen Butler was arrested Wednesday in Sacramento, California on charges, unrelated to the murder, of assault and elder abuse. He is now being held without bail on suspicion of murder.

On Wednesday morning, murder victim Fusako Petrus was on her daily morning exercise walk at the Highlands High School track with a friend, 61, when the two women were assaulted.

Neven Glen Butler, 18, has been arrested in the sexual assault and murder of Fusako Petrus, 86, and is also facing charges in a separate attack on a 92-year-old woman

Police say Butler beat an 86-year-old woman to death in a random attack as she walked around a high school track in Northern California on Wednesday

Butler first attacked Petrus' friend, according to police. The older woman heard the commotion and tried desperately to intervene, brandishing her walking stick.

Butler subdued the two women and sexually assaulted both of them before fatally beating Petrus, police said.

Cops got the 911 call and rushed to the scene, where Petrus was found dead despite lifesaving measures by first responders. The 61-year-old woman was treated for her injuries and is expected to recover.

The savage assault drew national attention as police launched a dragnet for the killer.

Later Wednesday afternoon, police responded to an assault on a 92-year-old woman about nine miles from Highlands High.

They found Butler at the scene and arrested him on charges of assault and elder abuse. The woman had injuries to her face, and was taken to a hospital for treatment.

Sacramento County Sheriff's Sgt. Tony Turnbull said Petrus and her friend routinely walked around the track

Homicide detectives with the Sacramento County Sheriff's Department say they can connect Butler to the scene of the sexual assaults and murder at Highlands High.

On Friday, police booked Butler on a murder charge, and he is not eligible for bail.

The brutal, unprovoked attacks on elder women shocked both the high school community and surrounding neighborhoods.

Petrus' neighbour, Julia Ortiz, told KCRA that she was a 'very sweet person' and was 'always smiling'.

She said: 'Who would think that somebody in their right mind would wake up in the morning and say "I'm going to harm people" - that's just not normal.'

'She was just a very sweet person, she was friendly and was always smiling and waving.'