They say early Saturday morning at the Bay Fair station, two men were arguing on the platform when one man, who was captured on surveillance video, walked up and punched one of them in the head.
The suspect is described as being about 5'8" to 5'10"... Medium build... With "shoulder-length dreadlocks" in a ponytail and a beard.
VIDEO: 'Remember Her' A tribute to Nia Wilson
Witnesses say the victim who fell to the ground and later died was a 47-year-old transient.
BART police also revealed an incident that turned fatal at the Pleasant Hill station.
Last Wednesday, a skirmish broke out between two men.
A 51-year-old Pittsburg resident left the train with a bloody lip and a cut to his knee.
VIDEO: What we know about deadly stabbing at Oakland's MacArthur BART station
He went to the hospital the next day on Thursday, not feeling well.
He was dismissed, but was found dead in his bed on Friday.
The 20-year-old man involved in that fight is being held at the county jail in Martinez on assault charges, but given the death, the charges could be upgraded.
BART police are asking anyone with information on these cases to contact them at 877.679.7000.
Go here for full coverage on the stabbing at Oakland's MacArthur BART station that took the life of 18-year-old Nia Wilson.
CONTINUING COVERAGE ON DEADLY BART STABBING:
- What we know about deadly stabbing at Oakland's MacArthur BART station
- 'Remember Her' A tribute to Nia Wilson
- Here's how you can help the family of BART stabbing victim Nia Wilson
- Chilling dispatch call describes terrifying fatal MacArthur BART stabbing
- 'He was wiping off his knife': BART stabbing victim recalls horrific attack that killed sister
- Heartbroken father demands justice after daughter stabbed at BART station
- 'Stand down': BART stabbing victim's godfather makes plea to black community
- TIPS: How to stay safe on BART
- Bart releases info on pair of homicides