US President Donald Trump has called for unity, after pipe bombs were intercepted on their way to the homes of Barack Obama and Hillary Clinton.
A potential explosive device and suspect package was also sent to the New York offices of the news network, CNN.
The Mayor of New York City, Bill de Blasio, has said the sending of the potential explosive device and suspect package to CNN "clearly is an act of terror".
The US Secret Service said it intercepted a 'suspicious package' on Tuesday addressed to Mrs Clinton in New York.
In Washington DC on Wednesday morning, another package was intercepted - this time addressed to the home of Mr Obama.
In a statement, the Secret Service said: "The packages were immediately identified during routine mail screening procedures as potential explosive devices and were appropriately handled as such.
"Both packages were intercepted prior to being delivered to their intended location. The protectees did not receive the packages nor were they at risk of receiving them."
BREAKING: Secret Service intercepts suspicious packages addressed to the residences of former President Obama and former First Lady Hillary Clinton. Working with our law enforcement partners on the investigation. Our Statement: https://t.co/lJdTn04KmI
— U.S. Secret Service (@SecretService) October 24, 2018
They added that "all available federal, state, and local resources" will be used to find those responsible.
Explosive devices were also sent to California congresswoman Maxine Waters and to former attorney general Eric Holder, whose package was reportedly addressed incorrectly and re-directed to Florida congresswoman Debbie Wasserman Schultz who had been named as the return address.
Meanwhile, the US news network CNN earlier evacuated its New York office due to a suspicious device.
The NYPD confirmed there was 'police activity' in the area after a suspicious package was discovered - while the network said the premises were evacuated out of an 'abundance of caution'.
CNN President Jeff Zucker said all the network's bureaus worldwide were searched as a precaution, but nothing was found.
The building has since re-opened.
Jeff’s final update: security sweep has concluded. Building reopened. https://t.co/I88Asy6Trm pic.twitter.com/wOPcektOPr
— CNN Communications (@CNNPR) October 24, 2018
CNN reporter Jim Sciutto said earlier the New York device was removed from the area in an NYPD containment unit.
Explosive device in transit now in NYPD containment unit. pic.twitter.com/Pt49kRm9UP
— Jim Sciutto (@jimsciutto) October 24, 2018
Speaking at the White House, President Trump said: "The safety of the American people is my highest priority. I have just concluded a briefing with the FBI, Department of Justice, Department of Homeland Security, and the US Secret Service.
"As we speak the packages are being inspected by top explosive experts, and a major federal investigation is now underway.
"The full weight of our government is being deployed to conduct this investigation, and bring those responsible for these despicable acts to justice.
"We will spare no resources or expense in this effort.
"And I just want to tell you that, in these times, we have to unify, we have to come together and send one very clear, strong, unmistakable message: that acts or threats of political violence of any kind have no place in the United States of America".
US Vice-President Mike Pence also condemned the "attempted attacks" against Mr Obama, Mrs Clinton and CNN.
We condemn the attempted attacks against fmr Pres Obama, the Clintons, @CNN & others. These cowardly actions are despicable & have no place in this Country. Grateful for swift response of @SecretService, @FBI & local law enforcement. Those responsible will be brought to justice.
— Vice President Mike Pence (@VP) October 24, 2018
New York City Police Commissioner James O'Neill said the CNN device appeared to be a "live explosive device."
The device has since been secured and removed for investigation.
While an envelope containing white powder was also sent to the office, which is still being investigated.
"The area has been swept with dogs and equipment and found no additional threats beside the white powder", Commissioner O'Neill said.
The NYPD said it is deploying resources around the city out of an abundance of caution.
New York City Police Commissioner James O'Neill (centre) addressing the media | Image via @NYPDnews on Twitter
Commissioner O'Neill said added: "If anyone sees something that looks suspicious, please call 911. Anyone with any information on this incident today, or about incidents in the New York area, please call 1-888-NYC-SAFE.
"We've seen this before, we've seen worse - and we will not be intimidated and we will bring these perpetrators to justice".
Mayor of New York City, Bill de Blasio (centre), addressing the media | Image via @NYPDnews on Twitter
New York Mayor Bill de Blasio said: "What we saw here today was an effort to terrorise; this clearly is an act of terror - attempting to undermine our free press and leaders of this country through acts of violence.
"I want to make very clear that the people of New York City will not be intimidated; that we're going to go about our lives undeterred - because the very concept of terrorism is to change us, and we will not allow that to happen".
"We at this time do not have other credible and specific threats against locations in New York City... That said, the NYPD and all of our partners have reinforced our very clear visible presence at key media locations around New York City and other important locations.
"You will see a lot more police presence - that should be reassuring to all New Yorkers".
It comes only days after officials in the US said an explosive device had been found in a mailbox at the New York home of George Soros - the billionaire who funds a number of progressive causes, and is a frequent target of right-wing criticism and conspiracy theories.
In a statement quoted by US media, White House Press Secretary Sarah Sanders said: "We condemn the attempted violent attacks recently made against President Obama, President Clinton, Secretary Clinton and other public figures.
"These terrorising acts are despicable, and anyone responsible will be held accountable to the fullest extent of the law."
Additional reporting: Jack Quann
