US President Donald Trump has called for unity, after parcel bombs were intercepted on their way to the homes of Barack Obama and Hillary Clinton.
A number of others have been delivered to US politicians and the CNN news channel.
The broadcaster's offices in New York were evacuated earlier, but have since-reopened.
CNN President Jeff Zucker has said all the network's bureaus worldwide were searched as a precaution, but nothing was found.
— CNN Communications (@CNNPR) 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.
President Trump says the United States needs to speak in a clear voice:
"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".
New York Mayor Bill de Blasio has branded the sending of a potential explosive device and suspect package to CNN as 'clearly an act of terror'.
Packages addressed to Mrs Clinton and Mr Obama were intercepted before delivery.
One of them was discovered about 50 kilometres north of New York City addressed to Mrs Clinton, while the other in Washington DC was sent to the Obama residence.
In a statement, the US 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."
It added: "The Secret Service has initiated a full scope criminal investigation that will leverage all available federal, state, and local resources to determine the source of the packages and identify those responsible."
We are aware of a suspicious package found in the vicinity of the Clinton residence in Chappaqua, NY, and our JTTF has engaged with our federal, state and local partners to investigate. As this is an on-going investigation, we will have no further comment at this time
— FBI New York (@NewYorkFBI) 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.
Mayor of New York City, Bill de Blasio (centre), addressing the media | Image via @NYPDnews on Twitter
Mayor 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".
US Vice-President Mike Pence has 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