The group of hackers known as Anonymous says it has hacked into about 70 mostly rural law enforcement websites in the US, a data breach that at least one local police chief said leaked sensitive information about an ongoing investigation.

The loose-knit international hacking collective posted a cache of data to the internet early on Saturday, including emails stolen from officers, tips that appeared to come from members of the public, credit card numbers and other information.

Anonymous said it had stolen 10 gigabytes worth of data in retaliation for the arrests of its sympathisers in the US and Britain.

Tim Mayfield, a police chief in Gassville, Arkansas, told the Associated Press that some of the material posted online – including pictures of teenage girls in swimsuits – was sent to him as part of an ongoing investigation. He declined to provide more details.

Mayfield's comments were the first indication that the hack might be serious. Since news of some kind of cyber-attack first filtered out less than a week ago, various police officials said they were unaware of the hacking or dismissed it as nothing to worry about.

Though many of the leaked emails appeared benign, some of the stolen material carried sensitive information, including tips about suspected crimes, profiles of gang members and security training.

The emails were mainly from sheriffs' offices in Arkansas, Kansas, Louisiana, Missouri and Mississippi. Many of the websites were operated by a media services hosting company in Mountain Home, Arkansas, and most, if not all, were either unavailable on Saturday or had been wiped clean of content. The company, Brooks-Jeffrey Marketing, declined to comment.

In a statement, Anonymous said it had leaked "a massive amount of confidential information that is sure to [embarrass], discredit and incriminate police officers across the US". The group said it hoped the disclosures would "demonstrate the inherently corrupt nature of law enforcement using their own words" and "disrupt and sabotage their ability to communicate and terrorise communities".

The group did not say specifically why these sheriffs' departments were targeted, but Anonymous members have increasingly been pursued by law enforcement in the United States and elsewhere following a string of high-profile data thefts and denial of service attacks – operations that block websites by flooding them with traffic.

Last month, the FBI and British and Dutch officials made 21 arrests, many of them related to the group's attacks on the internet payment provider PayPal, which was targeted over its refusal to process donations to WikiLeaks. Anonymous also claims credit for disrupting the websites of Visa and MasterCard in December when the credit card companies stopped processing donations to WikiLeaks and its founder, Julian Assange.

An internet security expert said Anonymous may have gone after the sheriffs' offices because the hosting company was an easy target. Dick Mackey, vice-president of consulting at SystemExperts in Sudbury, Massachusetts, said many organisations did not see themselves as potential targets for international hackers, causing indifference that could leave them vulnerable.

"It seems to me to be low-hanging fruit," he said. "If you want to go after someone and make a point and want to have their defences be low, go after someone who doesn't consider themselves a target."

As part of the information posted from US sheriffs' departments, Anonymous leaked five credit card numbers it said it used to make "involuntary donations". At least four of the names and other personal details published appeared genuine, although those contacted by the Associated Press said they did not know whether their financial information had been compromised.

Anonymous also posted several emails from police tipsters, many of whom had asked law enforcement not to use their names for fear of retaliation. One tipster wrote that his uncle was a convicted sexual offender who was homeless and hanging around a Walmart and other places where children were. Another tipster wrote to police that she and her neighbours could smell drugs coming from a house. Neither responded to emails requesting comment.

Most calls to more than two dozen affected sheriffs' offices went unanswered or were not returned on Saturday. Several confirmed that a cyber-attack had taken place, and some said they did not believe highly sensitive information had been leaked.

"At this point, other than emails ... there's really not any other critical information they could get their hands on," said John Montgomery, sheriff of Baxter County in northern Arkansas.

In Arkansas, the St Francis County sheriff, Bobby May, said his department and several others had been targeted in retaliation for the arrests of hackers who had targeted Apple Computer, among other companies.