London (CNN) -- Four people, all journalists or former journalists for Britain's Mirror Group Newspapers, have been arrested on suspicion of conspiring to hack voice mails, police in London said Thursday.
The investigation is mainly concerned with the Sunday Mirror newspaper and is focused on the period from 2003 to 2004.
"This conspiracy is being treated as a separate conspiracy to the two being investigated at the now defunct News of the World newspaper," London's Metropolitan Police said.
The News of the World was closed down in 2011 amid public uproar over the hacking of voice mails by employees of the tabloid, owned by the UK arm of Rupert Murdoch's News Corp.
A number of people face criminal charges as a result of investigations into alleged wrongdoing at the News of the World.
Trinity Mirror, the UK media giant which owns Mirror Group Newspapers, named two of those arrested Thursday as James Scott, the editor of The People and his deputy, Nick Buckley.
"We understand that two former employees were also arrested this morning," a Trinity Mirror spokesman said.
"The police are investigating allegations of phone hacking whilst they were on the Sunday Mirror during 2003 and 2004. We are cooperating with the police and we have no further comment to make at this stage."
Police earlier said the four people arrested Thursday "on suspicion of conspiracy to intercept telephone communications" were three men in their 40s and a woman aged 47.
They are being questioned at police stations in London, and a number of addresses are being searched, police said.
Mirror Group Newspapers publishes the Daily Mirror, the Sunday Mirror and The People, as well as Scotland's Daily Record and Sunday Mail.
In a separate development, a former Sussex police officer, James Bowes, was charged Thursday with misconduct in public office, UK prosecutors said.
"It is alleged that in 2010 Mr. Bowes provided information to The Sun newspaper in breach of the terms of his employment and was paid £500," a Crown Prosecution Service statement said.
The information related to three high-profile investigations being conducted by Sussex Police at the time, it said. He will appear in court later this month.
Parallel investigations were set up into claims of voice mail hacking and alleged illegal payments to police and public officials in the wake of the furor over abuses by the News of the World.