Mozambican jihadists launched a raid on Tanzania last week, killing an unknown number of people, in a daring cross-border attack which comes after an intensifying campaign at home.
Islamists began carrying out attacks in gas-rich northern Mozambique in 2017, pledging loyalty to the Islamic State a year later, and launching ever more brazen strikes such as the capture of a key port town in August.
Several small attacks blamed on Islamist extremists have targeted Tanzania, such as a series of killings of police and administrative officials in the eastern district of Kibiti in 2016 and 2017.
And in November last year, police blamed an attack on a border village that left six dead on insurgents from Mozambique.
Islamic State Central Africa Province (ISCAP), affiliated with the Islamic State group, last week claimed it had mounted an attack in Mtwara, near the Mozambican border, on 14 October.
"It's true that about 300 terrorists from Mozambique attacked our station at Kitaya in (southern) Mtwara last week and killed some people," said police chief Simon Sirro in a videotaped press conference viewed by AFP Thursday.
"Some of the suspects are from Tanzania who cooperated with others from outside Tanzania," he said on the island of Pemba.
"We will deal with anyone who kills Tanzanians. We are holding and questioning some suspects to get their full network."
Sirro linked the suspects to the 2017 attacks in Kibiti.
"Some of these killers crossed to Mozambique and now they want to come back. We will flush them out whether they are in Tanzania or any other country neighbouring country where they run to."
The shadowy Islamist group has terrorised northern Mozambique since 2017 and earlier this year openly declared its goal was to turn the gas-rich region into a caliphate.
"We want everyone here to apply Islamic law," a Kalashnikov-wielding group member told terrified residents in a video that appeared to have been shot in the port city of Mocimboa da Praia.
"We don't want a government from unbelievers, we want a government from Allah."
Locally they are known as Al-Shabaab, although they have no known links to the ruthless jihadist group of that name operating in Somalia.
However their attacks have increasingly been claimed by the Islamic State Central Africa Province (ISCAP), affiliated with the Islamic State group.
The group's attacks have killed more than 1,500 people and displaced 250,000, according to conflict data provider ACLED.
In August the group occupied Mocimboa da Praia, a major traffic hub for natural gas, 60km to the south of a liquefied natural gas (LNG) facility -- one of Africa's biggest single investment projects -- on Afungi peninsula.
The attack on Tanzania comes less than a week before presidential and parliamentary elections next Wednesday.