Folkehelseinstituttet (Norwegian Institute of Public Health) has found the same variant of E. coli O157:H7 in four people since August.
The agency was informed in mid-August of a child with E. coli O157:H7 infection that developed Hemolytic Uremic Syndrome (HUS) and was a resident of Møre og Romsdal.
It has detected the same bacterial strain in three adults with symptoms living in Hedmark, Buskerud and Hordaland.
Folkehelseinstituttet is working with Mattilsynet (Norwegian Food Safety Authority) and Veterinærinstituttet (Veterinary Institute).
Since the patients live in different counties, the investigation is coordinated by Folkehelseinstituttet.
Interviews are being done with sick people as well as sampling to try and find the source.
Folkehelseinstituttet said the investigation has so far not revealed any common source of infection and warned it was not always possible to find the source.
A total of 239 EHEC cases were reported in the country last year.
Norwegian foodborne outbreaks decline
Meanwhile, the number of outbreaks of communicable diseases decreased compared to previous years.
A total of 137 outbreaks with 2,185 affected people were notified to Folkehelseinstituttet in 2016.
About 20% of these (28) were suspected to be linked to contaminated food and 481 people were sickened.
The number has decreased compared to previous years (2015: 40 outbreaks and 483 ill).
The most commonly reported agent in 2016 was norovirus (10 outbreaks) followed by Campylobacter and Cryptosporidium (two each).
Buffet food caused the most cases, followed by vegetables and herbs, shrimps, crustaceans, shells, molluscs etc and fruits and berries.
One of the outbreaks was in relation to Salmonella Enteritidis in eggs from Poland in which 14 EU countries have been affected with Norway reporting 11 outbreak cases.
Norway said a significant proportion of food and waterborne infections registered in Meldingssystem for smittsomme sykdommer (MSIS) are due to infection during travel or stay abroad.
Good and bad pathogen news
The number of reported cases of campylobacteriosis is in line with previous years (2,307 in 2015 vs 2,317 in 2016) while Salmonella declined (928 in 2015 vs 865 in 2016).
A total of 969 Campylobacter cases were reported in Norway and 1,115 abroad with 233 having unknown infection sites. Of the domestic infection cases, 375 were hospitalised.
A total of 865 Salmonella cases were reported last year - 175 cases of domestic origin, 605 of foreign origin and 85 unknown with 240 people being hospitalised.
Listeria cases were 20, with 19 people hospitalised, compared to 18 cases in 2015.
EHEC cases have increased steadily over the last 10-15 years. In 2016, reported cases increased to 239, against 221 in 2015 and 151 in 2014.
Folkehelseinstituttet said a large part of this increase can be explained by introduction of new methods in routine diagnostics at several medical microbiological labs.
This means all submitted samples are investigated for a wide range of infectious agents at the same time, added the agency.