Eight people have been arrested in connection with the robbery in which Worcestershire sub-postmaster's son Craig Hodson-Walker was shot dead.
A West Mercia police spokesman said: "Four were arrested on suspicion of murder and attempted murder. Four other people were arrested on suspicion of assisting an offender."
Mr Hodson-Walker was shot in the chest on Friday morning as he tried to fend off four masked raiders at Fairfield Post Office and Stores in Fairfield, near Bromsgrove.
The 29-year-old lived and worked at the village Post Office alongside his parents, Ken and Judy, both 56. His father was shot in the leg during the attack.
The early morning arrests were made at six addresses across Birmingham and involved officers from West Mercia Constabulary and West Midlands Police. The suspects are being held at police stations across the West Mercia and West Midlands force areas.
The post office shooting happened at about 8.20am on Friday when a group of men armed with a handgun and sledgehammer burst into the store. They fled the scene in a silver Volkswagen Golf, which was stolen on New Year's Eve and found abandoned in a lane near the store shortly after the shooting.
Its registration plates had been changed for the robbery, police said.
The Post Office has offered a £50,000 reward for information leading to the arrest and conviction of Mr Hodson-Walker's killers.
A post-mortem examination confirmed that Mr Hodson-Walker died from a gunshot wound to the chest.
The shooting sent shockwaves through the quiet rural village of Fairfield, which is close to the M5 and M42 motorways.
Mr Hodson-Walker had recently become engaged to teaching assistant Lisa Bundy. Miss Bundy, from Ross-on-Wye, Herefordshire, has described her fiance as her "soulmate and best friend".
The police spokesman stressed that the investigation was still in its early stages and urged anyone with information to continue to come forward. He added: "Police would like to express their thanks to the public and the media for their information and co-operation so far during this ongoing operation."
A police spokeswoman later confirmed that three women and five men were arrested at various addresses in the south Birmingham area. Officers believe four men were involved in the robbery - three went into the store, while a fourth, the driver, waited with the silver Volkswagen Golf outside.
They fled the scene in the car, which was stolen from Solihull on New Year's Eve and had false number plates. Detectives said it was likely they had been caught in rush-hour traffic near the post office before the raid, and appealed for anyone who might have seen them to come forward.
Mr Hodson-Walker was born in Cardiff and spent his early years living in Barry, South Wales. The family later moved to Cornwall and then to the Forest of Dean, Gloucestershire, where they lived for 17 years.
Mr Hodson-Walker, described as a technological whizz and car enthusiast, attended Lakers School in Coleford, Gloucestershire, and then the Royal Forest of Dean College where he gained an A-level in IT.
The family moved to Bromsgrove to take over the post office in Fairfield in October 2003.