Seven people have been arrested in Vietnam, including three on terrorism charges, for bombing a police station in Ho Chi Minh City last month in a rare attack that injured three people.
The bomb was planted on a motorcycle and blew out the glass facade of the building near the site of violent protests over a proposed economic law just days before.
Two officers and a cook were injured in the explosion.
Officials Thursday did not make a direct connection to the protests and said only that it was a carefully orchestrated attack by alleged terrorists connected to a "foreign" group.
"The organisation of the attack... was with support from outside forces," HCMC Police Colonel Nguyen Sy Quang told reporters Thursday, without naming the alleged group.
"They had careful preparation and had tested explosives many times," he added.
Three of the men face terrorism charges, while another four face charges of providing explosives. At least three others are being sought in connection with the attack.
Authorities also seized dozens of detonators and 10 kilograms (22 pounds) of explosives that the group allegedly planned to use for further attacks.
The bomb attack came just 10 days after huge -- and rare -- rallies erupted in several cities across Vietnam last month where thousands gathered to protest against a draft law on special economic zones that would grant investors 99-year leases.
The proposed law sparked fears that China would take over the investment zones, igniting popular and deep-seated resentment against Vietnam's powerhouse communist neighbour.
China was not named in the draft law, and the government proposed reducing leases, but it was not enough to quell the anger.
Hundreds of uniformed and plainclothes police manned public parks and roads in Ho Chi Minh City the weekend after the rallies to block planned follow-up protests.
An American student Will Nguyen was detained in the demonstrations and is being investigated for "disturbing public order".
His family has rallied the US government to demand his release and hopes the case will be raised during a visit by Secretary of State Mike Pompeo to Hanoi on Sunday.
Scores were arrested in the nationwide protests on June 10.
A hardline administration in place since 2016 has been accused of quashing dissent and jailing activists at an unprecedented rate.
On Monday, a 44-year-old activist Le Anh Hung was arrested in Hanoi, his mother told AFP, saying authorities told her he was accused of tarnishing the prestige of Vietnam's leaders.