An envelope containing a suspicious white powdery substance has been opened at Trump Tower by staff working on Donald Trump's Republican presidential campaign.
The envelope caused a scare at the Republican front-runner's New York office but was later deemed to be harmless.
Five Trump staff members and a police officer who responded were temporarily isolated and evaluated. The substance was tested and, a few hours later, authorities said it was not hazardous.
The powder is undergoing further tests to determine what it is.
An envelope that contained a non-hazardous white powder and a threatening letter was sent to the apartment of Mr Trump's son, Eric Trump, who has campaigned for him, in March.
The handwritten note, postmarked from Boston, said: "If your father does not drop out of the race, the next envelope won't be a fake."
Two days later, a threatening letter was sent to Mr Trump's sister Maryanne Trump Barry, a judge who sits on the US Circuit Court of Appeals for the 3rd Circuit in Pennsylvania. The FBI said at the time it was working alongside the Secret Service and the Marshals Service to investigate.
Mr Trump has been criticised by some political rivals and voters for his comments on topics including women, refugees and immigrants, such as when he said some Mexican immigrants in the US illegally are "rapists".
He is due to speak at California's Republican Party convention later on Friday. Presidential nomination rival John Kasich will also speak to the convention on Friday while Ted Cruz and his running mate Carly Fiorina will appear on Saturday.
The billionaire businessman rarely speaks to Republican establishment groups and has railed against what he calls a rigged party system that governs the nomination.
He began his trip to California on Thursday with a rally at the Orange County Fairgrounds in Costa Mesa, where he filled the Pacific Amphitheatre to its capacity of around 18,000.
The candidates are seeking to galvanise supporters, sway undecided party members or poach them from rival campaigns ahead of the state's primary on June 7.
It is possible that California, home to 172 delegates, could be crucial in Mr Trump's bid to reach the target of 1,237 needed to clinch the Republican nomination. Mr Trump currently has 994 delegates, Mr Cruz has 566 and Mr Kasich has 153.
Thad Kousser, who teaches political science at the University of California, San Diego, said: "Donald Trump is not going to agree with every member of this audience on every issue but he remains the rock star of this presidential race."
But Mr Cruz has an advantage due to his organisation in the state since last summer.
Mr Kasich, the holder of one primary victory, his home state of Ohio, is looking to make inroads in California districts that could be favourable to his more moderate credentials.