Investigators said the suspect, Floyd O'Neal, 31, disguised himself during the incident, and has admitted striking Manzanita Community School Principal Eyana Spencer with a pistol. O'Neal said he did not remember taking her purse, Sgt. Kevin Johnson said.
O'Neal is scheduled to be formally charged and arraigned today.
Police linked O'Neal to the attack after the pistol was seized from him following an incident July 5. Spencer's DNA was found on the weapon, Johnson said.
O'Neal, free on $90,000 bail in the July case, was being held Wednesday on suspicion of assault with a deadly weapon, assault of a school administrator in performance of her duties, and robbery.
O'Neal, fired by the school district July 17, was already charged with six felony counts for the July 5 incident, including numerous weapons counts and evading police.
Spencer was attacked in her office by a man wearing a hat and sunglasses and with a dreadlocks hairstyle, which is thought to have been a wig, Johnson said.
Her purse, containing $11, was taken, but was found later in North Oakland.
Police thought O'Neal might be a suspect because of disciplinary problems he was having at the school.
After his arrest, O'Neal claimed he wore a disguise to avoid narcotics dealers in the area he said were looking for him, police said.
He also told Johnson and Sgt. Mike Gantt that he attacked Spencer because he was in the process of being disciplined and probably fired for poor work performance, police said.
An 11-year veteran of the district, O'Neal had worked at the school for several years. Spencer had been principal there for three years.
"We're relieved the suspect was apprehended," said district spokesman Troy Flint. "Of course, it's distressing to realize it's somebody in the OUSD family."
Flint would not discuss O'Neal's record in detail or the reasons for his firing, but acknowledged the custodian has a history of discipline issues.
"We're hopeful this helps bring some peace of mind to the community as well as closure to what's been a very difficult situation," Flint said.
He was quick to add that the district is continuing a comprehensive review of its security measures, a need highlighted earlier this week when three other school custodians were arrested after being accused of theft and possession of stolen school property.
Two of those custodians, Kenneth Hill, 43, and Sylvester Lawson, 52, have been charged. Prosecutors declined Wednesday to charge the third custodian, pending further investigation.
"The review will continue until we're satisfied we've found the proper answers. We're looking at every facet related to security: hiring practices, reporting discipline, securing grounds and premises. It's an A-to-Z review," Flint said.
The key break in the O'Neal case came July 5 when CHP officers made a traffic stop on O'Neal at East 33rd Street and Park Boulevard, across the street from Oakland High School.
CHP officers say they saw the assault pistol in the car and O'Neal sped off. He was eventually caught and arrested by Oakland police, who recovered the pistol and the dreadlocks wig from the vehicle, Johnson said.
The police crime lab later told Johnson the principal's DNA was on the gun.
O'Neal was arrested at his East Oakland home Tuesday by the department's Police and Corrections Together team.
He has no prior record. His bail is set at $100,000.