This story is continuously being updated.

City officials don't know who triggered Dallas' outdoor warning sirens late Friday, but they do know how it was done — by broadcasting a few tones, via either radio or telephone signal.

In other words, there was no computer hack.

"It's a radio system, not a computer issue," Dallas City Manager T.C. Broadnax said Monday morning.

The city's outdoor warning sirens had to be manually shut down and turned back on late Sunday, with "immediate fixes" intended to prevent the type of incident that woke up — and shook up — much of the city Friday night, according to Broadnax.

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"As we brought the system back up, some encryption was added as part of our process to prevent this type of error from occurring going forward," he said.

City officials said late Monday that the system was purchased a decade ago and that encryption was not part of the original deal with the vendor for one simple reason: No one at City Hall knew something like this was possible.

"No one had ever seen anything like this," said Sana Syed, the city's spokeswoman. "No one had ever seen anyone able to break into the entire system and impact every signal."

Syed said that within 48 hours of Friday's incident, "the system was not only encrypted, but other safeguards were taken."

The city is not providing further details on those security upgrades. But the fix could be relatively short-term anyway, as city officials investigate replacing the system that triggers the sirens.

Dallas' emergency warning sirens, like many across the country, are radio-controlled and activated via encoded transmitters that act like pagers, sending tones to receivers attached to each siren. In Dallas' case, security experts say, it appears that someone took control of the frequency and transmitted the tones that turned on all 156 sirens across the city.

Broadnax said he was "leery about [sharing] how these systems work," citing security concerns. He would confirm only that "it's a tonal-type system."

In Moore, Okla. — a town in Tornado Alley that relies on sirens for weather emergencies — a base radio station is set up in a secure location. An employee activates the system, and the radio sends out an encoded message via the airwaves. The sirens then receive the message, decode it and start to wail, said Gayland Kitch, director of emergency management for Moore.

It's likely that whoever activated Dallas' 156 sirens did so by compromising the city's infrastructure and gaining access to the codes, said Jeff Schilling, who has a background in military communications and is now chief security officer of Armor, a cloud security company based in Richardson.

"If I were doing the investigation, the first thing I would be looking for is former and current employees who have extensive knowledge of how the system works, or contractors or maintenance people who have worked on the system," Schilling said. "It's very unlikely that someone did this with their own radio gear."

One of Federal Signal Corp.'s sirens, this one at the corner of Gaston Avenue and North Good-Latimer Expressway in Dallas. (Jae S. Lee/Staff Photographer)

Illinois-based Federal Signal Corp. provided Dallas with its outdoor warning system, which the City Council has spent millions in bond money on, upgrading and expanding it since 2007. An August 2011 council resolution shows that the contract with Federal Signal is for upward of $3.3 million.

In November, the council awarded Michigan-based West Shore Services a separate six-year, $567,368 contract to maintain and repair all of the city's emergency sirens.

In a statement sent to The Dallas Morning News late Monday, Federal Signal Corp. said the company is "actively working with the Dallas Office of Emergency Management to determine the cause of the unintended activation." Officials at West Shore Services were not immediately available for comment.

A similar incident occurred in Lemont, Ill., in 2012, when an unauthorized signal triggered that town's seven radio-controlled sirens. Officials there employed a similar fix, encrypting the signal.

On Saturday, city officials said they believed the incident originated locally. Broadnax said the Dallas Police Department is leading the investigation and that the FBI and Federal Communications Commission, which governs — and recently narrowed — the public safety frequency, have been brought in to assist.

A map of Dallas' outdoor warning system

"We are not sure what specifically the FCC is going to do and can do in terms of their resources in how they address this case," he said. "But we are working with the FBI to see if we can find the individuals who committed this crime, and as the mayor indicated, to make every effort to prosecute them to the fullest extent of the law."

In a statement to The Dallas Morning News on Monday afternoon, Will Wiquist, FCC deputy press secretary, said, "While we cannot comment further on this specific case at this time, we take very seriously allegations regarding activities that may interfere with public safety."

FBI officials wouldn't confirm or deny their involvement in the investigation.

Broadnax said that any extensive overhaul of the outdoor warning system wouldn't be inexpensive or immediate. Updates will be provided to the City Council, in public and behind closed doors, in the coming days.

Well, ya finally woke up the kid in a panic. pic.twitter.com/bN0WA9i84w — Robert Wilonsky (@RobertWilonsky) April 8, 2017

For now, he said, "we have resolved access and the ability for others to do what they had done that caused the incident this weekend." Sooner than later, he added, his office will determine "whatever is the best system is out there and make recommendations to council."

In September, City Auditor Craig Kinton told the council his office planned to review how vulnerable the city is to outside interference as part of this fiscal year's audit plan. Broadnax said Monday that it's likely the city will hire an outside consultant to assess all of its security measures, from the water department to 911.

"I am looking forward to advancing a council discussion about looking at other systems," he said, "and ensuring they are not accessible to hacks or other outside malicious behavior."

An action item will be presented to and discussed with the City Council on Wednesday.

Friday night's incident also alerted city officials to another problem: its inability to tell residents when they shouldn't panic. The sirens began blaring around 11:40 p.m., and it took the city more than an hour to issue a statement, which it did via social media. Initially, the city said it believed a "malfunction" triggered the sirens.

Some 4,400 calls poured into the city's already understaffed 911 call center, resulting in significant delays.

Broadnax, whose first day as city manager was Feb. 1, said the incident made it clear the city needs "more comprehensive protocols" when dealing with these types of situations.

That includes "touching base with our own City Council as they get phone calls," he said. "That was apparent. We're going to have to work on that, along with how we share with the media all forms of communication."

Federal data shows a marked uptick in crimes attempted against critical infrastructure, from traffic signals to a small dam in upstate New York. On Monday, U.S. Rep. John Ratcliffe, the North Texan who chairs the House's Cybersecurity and Infrastructure Protection Subcommittee, said cybersecurity issues are a top priority.

"The recent hacking attack in Dallas demonstrated the disruption that can be caused by those seeking to interfere with our way of life," he said. "It's one of the reasons I held a Homeland Security hearing in North Texas to examine cyberpreparedness and response at the local level, and why I'm committed to continuing this engagement going forward."

Staff writer Sarah Mervosh contributed to this report.