Police in Dungannon are investigating an allegation of electoral fraud in connection with postal and proxy voting applications.
It is understood the police probe was launched after concerns were raised by the Electoral Office.
Meanwhile, the Alliance Party has complained to the police about phoney leaflets which bear the party's colours and urge voters to back the UUP.
The Ulster Unionist Party has distanced itself from the leaflets.
Alliance leader David Ford is blaming a "dirty tricks campaign designed to help the UUP".
"Many people have told us that they have received a leaflet printed in
Alliance colours of yellow and blue, headed 'Thinking of Voting Alliance'," he said.
"It is clearly intended to confuse Alliance supporters."
Meanwhile, a record number of voters in Northern Ireland have applied for "absent votes" for the 5 May poll.
More than 33,000 applications have been received from people who claim they cannot vote in person.
The figure includes both postal and proxy votes and is about 10,000 up on the 2004 European election and the 2003 assembly election.
Voters can claim an absent vote due to sickness, if they have to be away on business or are going on holiday.
But the late spring election date of 5 May is well outside Northern Ireland's main holiday season of June to August.
Despite this, Northern Ireland's electoral authorities are reportedly fairly confident there will be a low incidence of fraud related to the postal votes.
There has been a wave of concern about fraud in other parts of the UK.
However, the electoral fraud laws enable the authorities in Northern Ireland to scrutinise an applicant's signature and to check their date of birth and national insurance number against their database.
Some parties believe there still may be some loopholes.
The SDLP say they have come across cases of people who have given what they believed was a postal application form to a political party, but have then found the form switched to an application for a proxy vote, to be cast by another person.
The Electoral Office placed adverts in Northern Ireland's local newspapers on Friday telling people how to use their postal votes and warning them to keep their papers safe and secret.
Chief Electoral Officer Dennis Stanley said his office strove to ensure that absent voting was not exploited.
"We are always concerned about fraud," he said.
"We want to keep as vigilant as possible, and we want to make sure every person has the opportunity to cast their vote in a fair and free way and that no-one interferes with it, so absent voting is a particular area we pay attention to."
The votes have to be returned by 2200 BST on polling day.
Although there is no breakdown by seat of how many absent votes have been applied for in each constituency, the counting centres have the following figures:
3,264 between the four Belfast seats
7,692 between Upper Bann, Newry and Armagh, South Down and Lagan Valley
9,819 between West Tyrone and Fermanagh South Tyrone
1,534 between East Antrim and South Antrim
5,350 between North Antrim and Mid-Ulster
4,000 between Foyle and East Londonderry
1,784 between Strangford and North Down.