LONDON -- Britain's phone hacking scandal advanced closer to another leading newspaper group on Thursday when Trinity Mirror said the police were investigating whether the company was criminally liable for allegations of "unlawful conduct" by previous employees of the Sunday Mirror tabloid.

"The group does not accept wrongdoing within its business and takes these allegations seriously," Trinity Mirror, which also publishes the left-leaning Daily Mirror tabloid and Sunday People, said in a statement. "It is too soon to know how these matters will progress, and further updates will be made if there are any significant developments."

In March, the police said that four Mirror group journalists had been arrested in South London on suspicion of conspiracy to intercept telephone communications. The journalists were not identified by name. Scotland Yard said they were three men ages 40, 46 and 49, and a 47-year-old woman. British news reports at the time said the four were all senior current or former editors, including the editor and deputy editor of the tabloid Sunday People and the former editor and former deputy editor of The Sunday Mirror.

Thursday's announcement indicates that the police are also investigating whether the employer of the arrested journalists bears corporate responsibility for any wrongdoing.

The hacking scandal has largely embroiled British newspapers in Rupert Murdoch's empire. In July 2011, Mr. Murdoch closed The News of the World, a tabloid, after disclosures that its employees had hacked into the cellphone messages of a teenager, Milly Dowler, who had been abducted and was later found murdered.

Two former editors and several ex-employees of Mr. Murdoch's British newspaper subsidiary have been charged and trials are expected to start later in the year. All have denied wrongdoing.

The hacking scandal led to an array of investigations, one of which, the Leveson inquiry, concluded that Britain needed a new form of press oversight with statutory underpinnings.

One of the witnesses at the inquiry, led by Lord Justice Sir Brian Leveson, was the CNN talk show host Piers Morgan, who was editor of The Daily Mirror from 1995 to 2004. He told the investigation that he knew no one who hacked phones.

Other witnesses testified that phone hacking was rife at The Mirror, but Mr. Morgan repeatedly testified that it was not and that he knew nothing about it.

The Sunday Mirror, according to recent industry statistics, had a circulation of around one million in August, but The Sun on Sunday, which replaced the shuttered News of the World, led the Sunday tabloid market with sales of 1.9 million, followed by The Mail on Sunday with around 1.6 million.

First Published: September 12, 2013, 10:00 p.m.