A fourth man has been arrested over the kidnap and killing of 59-year-old British aid worker Margaret Hassan, the UK embassy in Iraq has said.
Items found during the raid link the suspects to Mrs Hassan
On Sunday, three men were among a group detained when Iraqi and US forces raided homes south-east of Baghdad.
Police said they found ID, clothing and a handbag belonging to Dublin-born Mrs Hassan, who was abducted on 19 October.
A video showing Mrs Hassan's apparent death was released a month later but her body has never been recovered.
Iraqi police said earlier that five suspects had confessed to the killing of Mrs Hassan but there is no confirmation of this.
The police raids in Madain, a lawless area 25 miles south-east of Baghdad where recently more than 50 people have died in a series of sectarian killings, followed a series of car bombs on Friday.
A Foreign Office spokesman said: "We can confirm that four people are in Iraqi custody in connection with the Hassan case.
"The arrests were made after articles were found that established a direct connection with Margaret Hassan."
Mrs Hassan was an Iraqi/British dual national and had lived in Iraq with her Iraqi husband, Tahseen.
She began working for Care International soon after it began operations in Iraq in 1991, and provided humanitarian relief to the most needy Iraqis in a professional career spanning more than 25 years.
When she was kidnapped, Mrs Hassan had been head of the charity's operations in the country for 12 years and had lived in the country for 30 years.
Also on Sunday militants issued a tape of an Australian man who has been taken hostage in Iraq.
The man, who identified himself as 63-year-old Douglas Wood, appealed to the US, British and Australian governments to pull troops out of Iraq.
"My captors are fiercely patriotic. They believe in a strong, united Iraq looking after its own destiny," he says in the tape, whose authenticity could not be confirmed.