"It is horrific and unprecedented," acting Union County Prosecutor Grace Park told reporters, CNN affiliate WCBS reported.
"I don't know how you would define a mass killer. All I would say is that we have never seen one individual commit this type of shooting spree, murder spree in such a short period of time here in this county."
Todd was arrested in Allentown, Pennsylvania, on Monday in connection with the shooting deaths of two people in Allentown and another in Easton, Pennsylvania, according to a statement from Park's office.
The motive for the two-state killing spree is under investigation.
West was already wanted for the shootings in the neighboring state of New Jersey when Pennsylvania authorities picked him up, according to prosecutors, CNN affiliate WFMZ reported.
In Pennsylvania, Todd and two other men are accused of fatally shooting Kory Ketrow, 23, in Easton about 3:15 a.m. Sunday before heading to Allentown and gunning down Trevor Gray, 21, and Francine Ramos, 32, authorities said, according to WFMZ.
Northampton County District Attorney John Morganelli told the station that the shootings were random and the victims may not have known the killers.
Todd and two alleged accomplices -- Robert Jourdain, 20, and Kareem Mitchell, 23 -- are being held without bail in Pennsylvania, WFMZ reported. It was not clear whether they have attorneys.
A week before the Pennsylvania bloodshed, authorities said, Todd allegedly fatally shot three people and wounded another in Elizabeth, New Jersey.
The multiple murder and weapons charges West faces in his home state stem from the June 25 shooting deaths of Elizabeth residents Dennis Vega, 28, Jamil Payne, 29, and Kelvin Nelson, 32, according to Park. He also faces a count of attempted murder and weapons offenses in the shooting of Richard Marte, 24, in Elizabeth that same day.
The New Jersey violence started about 2:30 a.m. on June 25, prosecutors said. Police received calls from people who heard gunshots and saw a man down in the street. The victim was later identified as Vega, who was shot several times, according to Parks.
At about 9:50 p.m. the same day, Elizabeth officers again responded to reports of gunfire and a man lying on the ground, Parks said. That was Payne, who was shot multiple times.
An hour later, with investigators still at the scene of Payne's shooting, police received calls about shots fired less than a mile away, prosecutors said. Nelson was later found dead from numerous gunshot wounds.
Less than 10 minutes after reports of Nelson's shooting, police were called to yet another shooting, prosecutors said. Marte suffered severe gunshot wounds but is expected to survive.
Surveillance cameras captured Marte's shooting, WCBS reported.
Parks said one common thread emerged from the investigations: a white, 2013 Buick LaCrosse was in the immediate vicinity of each shooting. Authorities also learned that the same .38 caliber, six-shot revolver was used in all of the shootings.
In the last week, New Jersey authorities were able to link West to the deaths of Payne, Vega, Nelson and Michael Thompkins, 29 -- who is the suspect's cousin and was fatally shot on May 18, also in Elizabeth, according to Parks.
No court date has been set in Union County, and the bail on all the New Jersey charges is set at $5.5 million, according to Parks.
Park said she doesn't believe the burst of violence was gang-related, WCBS reported.