A manhunt is underway.
A funeral procession for New Kensington officer Brian Shaw arrived in Lower Burrell Saturday afternoon.
The shooting happened around 8 p.m. ET Friday in New Kensington, about 18 miles from Pittsburgh.
"There are a lot of heavy hearts and they're going through a tough process, but yes staying very focused on their job which is to continue this investigation, find the person who killed Officer Shaw and bring him to justice", Pennsylvania Trooper Steven Limani said.
The FBI and Bureau of Alcohol, Tobacco, Firearms and Explosives are offering up to $40,000 for any information that would lead to Holt's arrest.
Klein said the traffic stop resulted in a chase on foot before Shaw, 25, was shot.
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Limani would not discuss other details of the case, including the reason for the traffic stop, saying it could compromise the ongoing investigation.
Holt faces charges of murder and murder of a police officer. It also remains unclear how many shots were fired, or whether one of them came from the officer's gun.
Police have not yet said if Shaw discharged his weapon.
Officers from Westmoreland County and neighboring Allegheny County, as well as Pittsburgh police, combed the area overnight and processed the scene, CNN affiliate KDKA reported.
Pittsburgh Steelers coach Mike Tomlin tweeted his condolences to Shaw's family and New Kensington police.