(CNN) Police and CNN have issued an all-clear message after the network's New York offices and studios were evacuated Thursday night because of a phoned-in bomb threat.

"The NYPD has now given us the all clear, and employees have been permitted to return to the building,"CNN Worldwide president Jeff Zucker said in an internal memo to staff. "The building is secure and safe for everyone to return in the morning."

"We appreciate the swift action by the local authorities, and the patience and professionalism of all the employees who were impacted."

The threat was not substantiated, the New York Police Department tweeted.

The threat came Thursday night when a caller indicated there were five devices in the building, according to a law enforcement source.

Several fire alarm bells rang inside CNN's New York newsroom, signaling an evacuation after the call was received shortly before 10 p.m., the company's security said.

Staffers evacuated the CNN New York offices in the Time Warner Center building and Don Lemon's "CNN Tonight," which had been on a commercial break, was taken off the air.

The network temporarily went to taped programming for about 30 minutes before going live from the street while police investigated.

"We were told to evacuate the building and to do it as soon as possible. We grabbed what we could and got out of the building and now, we are standing outside of the building," Lemon said.

Police and firefighters arrived around 10:30 p.m. to the Time Warner Center in New York.

"All of the employees, all the people who were in the building as we were doing our live broadcast are out of the Time Warner Center now," he added.

Numerous police and fire department trucks blocked all vehicle and pedestrian traffic on West 58th Street between 8th and 9th Avenue due to a police investigation at Columbus Circle, where the Time Warner Center is located, the New York Police Department tweeted.

A bomb squad was on standby outside the building as police officers search the offices, a law enforcement source said.

The network's bureau was evacuated in October after a package with an explosive device, addressed to former CIA Director John Brennan, was discovered, officials said.

That package was among more than a dozen improvised explosive devices sent to prominent Democratic politicians and donors around the country in October. None of the devices detonated, and no one was injured.

Suspect Cesar Sayoc, 56, was charged with five federal crimes and is currently in custody waiting for trial. If convicted, he could receive up to 48 years in prison.