Police continue to search the crime scene in Birstall, West Yorkshire, England, today, a day after Labour Party legislator Jo Cox was murdered in the street outside a library in her northern England constituency. The suspect, 52-year-old Thomas Mair of West Yorkshire, was arrested Thursday.

BIRSTALL, England — Campaigning in Britain’s European Union membership referendum halted and normal political life was suspended today as the country absorbed the daylight slaying of lawmaker Jo Cox with shock — and worry that the political fury unleashed by the EU campaign was somehow connected to the killing.

As Prime Minister David Cameron headed to the small town in northern England where the attack unfolded, a U.S. civil rights group said the man suspected of the gun and knife attack had links to an American white supremacist organization. The Southern Poverty Law Center said it has records showing Thomas Mair was a supporter of the National Alliance. The center said Mair purchased a manual from the group in 1999 that included instructions on how to build a pistol.

On its website, the center published copies of receipts showing that a Thomas Mair of West Yorkshire — the county where Cox and her suspected killer both lived — bought publications including “Chemistry of Powder and Explosives” and “Improvised Munitions Handbook.”

The address on receipts corresponded to a house that was cordoned off by police tape and guarded by uniformed officers.

A Thomas Mair of Batley — the town where the suspect lives — was named as a former subscriber to pro-Apartheid publication SA Patriot. In 2006, the online newsletter of the far-right group Springbok Club said Mair was “one of the earliest subscribers and supporters of SA Patriot.”

Mair, 52, was arrested Thursday on suspicion of killing Cox, who was shot and stabbed outside a library in her northern England constituency. The suspect’s brother, Scott Mair, told reporters his brother had a history of mental illness, but was not violent.

Witnesses said Cox, a 41-year-old Labour Party legislator, was attacked by a man with a homemade or antique-looking gun. Clarke Rothwell, who runs a cafe near the scene of the slaying, said the assailant shouted “Britain first” or “put Britain first” several times. Britain First is the name of a far-right group, which said it had no connection to the killing.

Cox was a former aid worker who had championed the cause of Syrian refugees and campaigned for Britain to stay in the EU when it votes in a referendum Thursday. The referendum has sparked an intense debate about immigration and Britain’s place in the world. “Leave” campaigners have said voters should quit the EU to take their country back from bureaucrats in Brussels and curb large-scale immigration from other EU nations.