WASHINGTON, May 30 (Reuters) - - An unidentified hacking

group launched a massive cyberattack on a telecommunications

company in the U.S. heartland late last year that disabled

hundreds of thousands of internet routers, according to research

published Thursday.

Security analysts with Lumen Technologies' ( LUMN ) Black

Lotus Labs discovered the attack in recent months and reported

on it in a blog post.

The October incident, which was not disclosed at the time,

took more than 600,000 internet routers offline. Independent

experts said it appeared to be one of the most serious

cyberattacks ever against America's telecommunications sector.

The researchers said the hackers installed malicious

software that disrupted internet access from Oct. 25 to 27

across numerous Midwest states. The analysts found the malware,

which continued circulating, on the internet months later

through certain file links that the hackers left visible.

The report did not name the company that was attacked.

Nor did Lumen attribute the hack to a particular country or

known group. The researchers said the saboteurs used common

methods which made them harder to identify.

The internet routers were disabled when a malicious firmware

update sent to the company's customers deleted elements of the

routers' operational code, making them effectively inoperable.

Exactly how the firmware update was shipped to users was

unclear.

"We assess with high confidence that the malicious firmware

update was a deliberate act intended to cause an outage,"

Lumen's report said. "Destructive attacks of this nature are

highly concerning, especially so in this case."

A comparison of details and event descriptions in the Lumen

report with internet outages on the dates of the attack pointed

to one entity: Arkansas-based internet service provider

Windstream.

A spokesperson for Windstream declined to comment as did the

FBI. The National Security Agency and Homeland Security

Department referred inquiries to the FBI.

The researchers described the potential consequences from

the attack as serious.

"A sizeable portion of this ISP's service area covers rural

or underserved communities; places where residents may have lost

access to emergency services, farming concerns may have lost

critical information from remote monitoring of crops during the

harvest, and health care providers cut off from telehealth or

patients' records," the researchers wrote.

There are few public signs of the incident. On the social

media platform Reddit, self-identified Windstream customers

posted complaints about a strange outage beginning around Oct.

25, the date noted by Lumen.

The Reddit users described how their routers would not

connect to their internet provider so they could not access the

internet. The users said Windstream was requiring them to return

their disabled routers for new devices because a remote fix did

not seem possible.

It was not clear if the FBI, which is in charge of

investigating U.S. cybercrimes, was notified of the hack. But

private companies often elect not to disclose such incidents.