A 50-year-old woman arrested on suspicion of conspiracy to defraud the local election process has been released on bail.
She was held on Wednesday following a raid in Birmingham where a large number of postal voting forms were recovered, West Midlands Police said.
The suspect was arrested ahead of next week's local elections at an address in Ronald Road in Bordesley Green.
She was released on Wednesday night pending further inquiries.
The BBC has learned the arrested woman is the wife of a Lib Dem candidate standing in the forthcoming elections.
Although she was arrested in Bordesley Green, her husband is not standing in that ward.
A second raid was carried out on Wednesday by the force's economic crime unit in nearby Hob Moor Road, where further material was recovered.
'Prevent and deter'
Electoral officials nationwide want to avoid a repeat of allegations of postal vote fraud which surrounded local elections in Birmingham in June 2004.
A subsequent election court hearing found that ballot-rigging in two of the city's wards "would have disgraced a banana republic".
An Electoral Commission spokeswoman said the responsibility for investigating allegations of fraud lay with the police.
She added: "But we have been working with the police to produce practical guidance on how to prevent and deter electoral fraud.
"We are pleased that the police are treating allegations of fraud with the seriousness that a criminal offence merits."
This year, the council had issued 59,962 postal voting forms, 10,000 less than the 2004 figure.
In a separate case Scotland Yard said officers were looking at suspicious activities in seven London boroughs amid claims of vote-rigging.