HOUSTON — Computer systems in 22 small Texas towns have been hacked, seized and held for ransom in a widespread, coordinated cyberattack that has sent state emergency-management officials scrambling and prompted a federal investigation, the authorities said.

The Texas Department of Information Resources said Monday that it was racing to bring systems back online after the “ransomware attack,” in which hackers remotely block access to important data until a ransom is paid. Such attacks are a growing problem for city, county and state governments, court systems and school districts nationwide.

By Tuesday afternoon, Texas officials had lowered the number of towns affected to 22 from 23 and said several government agencies whose systems were attacked were back to “operations as usual.”

The ransomware virus appeared to affect certain agencies in the 22 towns, not entire government computer systems. Officials said that there were common threads among the 22 entities and that the attacks appeared not to be random, but they declined to elaborate, citing a federal investigation.

It was unclear who was responsible. The state described the attacker only as “one single threat actor.”

Elliott Sprehe, a spokesman for the information resources department, declined to provide further specifics or release the names of the towns affected because of the “potential for further attacks.”

He declined to say if any of the towns had paid up.

“It’s limited to just a handful of areas,” Mr. Sprehe said. “It’s not disparate throughout the state.”

The attack began on Friday morning. Later that day, Gov. Greg Abbott ordered the second-highest level of alert in the state’s emergency-response system, classifying the attack as a Level 2 Escalated Response, meaning that the scope of the incident had reached beyond what local responders can manage.

“Governor Abbott is also deploying cybersecurity experts to affected areas in order to assess damage and help bring local government entities back online,” Nan Tolson, a spokeswoman for the governor, said in a statement.

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