Washington: The Republican National Committee's Washington headquarters was briefly evacuated on Wednesday as police investigated two vials of blood that had been addressed to former President Donald Trump following the presumptive presidential nominee's takeover of the national party apparatus.

Hazardous materials teams were called in after the vials were discovered, according to the US Capitol Police, who said they would continue to investigate. It was unclear if anyone came into contact with the blood and to whom it belonged.

The vials were addressed to Trump, according to a person familiar with the situation but not authorised to speak about it publicly. It was unclear if any message accompanied the vials explaining why they were sent.

Spokespeople for the RNC and the US Secret Service did not immediately return messages seeking comment. The Metropolitan Police Department and the local fire department referred comments to the Capitol Police.

Earlier Wednesday, the Capitol Police issued a statement advising people to avoid the block where the RNC is located, a short walk southeast of the Capitol.

The House sergeant at arms, the US House of Representatives' chief law enforcement and protocol officer, sent out information advising traffic restrictions in the area “due to law enforcement activity at the RNC.”

Trump's handpicked leadership — including his daughter-in-law, Lara Trump, as the party’s national vice chair and former North Carolina GOP Chairman Michael Whatley as RNC chairman — recently took over the RNC, completing his takeover of the national party as he closes in on a third straight GOP presidential nomination.

A Trump campaign senior adviser, Chris LaCivita, has taken over as the RNC chief of staff.

Wednesday's situation comes less than two months from the Republican National Convention in Milwaukee, where Trump is slated to become the party's official 2024 nominee and significant protests are expected.

According to a letter sent last month to the Secret Service, RNC counsel Todd Steggerda asked officials to keep protesters back farther from the site than had been originally planned, arguing that an existing plan “creates an elevated and untenable safety risk to the attending public.”