The suspected ringleader of Saturday's failed coup plot in Ethiopia's Amhara region is on the run, a senior security official has told the BBC.
Four top officials, including the army chief, were killed while thwarting the coup, officials said.
Flags are flying at half-mast after the federal government declared a day of mourning to mark the deaths.
Prime Minister Abiy Ahmed has urged Ethiopians to unite against "evil" forces set on dividing the country.
A heavy contingent of pro-government forces have been deployed in Amhara's capital, Bahir Dar, and the federal capital, Addis Ababa.
The US state department has warned its staff in Addis Ababa to stay inside.
Ethnic violence has hit Amhara and other parts of Ethiopia in recent years.
Since his election last year, Mr Abiy has moved to end political repression by releasing political prisoners, removing bans on political parties and prosecuting officials accused of human rights abuses.
Africa's oldest independent country, Ethiopia is also the continent's second most populous (after Nigeria), with 102.5 million inhabitants from more than 80 different ethnic groups.
A transfer hub for long-haul air travel, it has one of the fastest-growing economies in the world, but a vast number of young Ethiopians are without work.
Who is the alleged ringleader?
Mr Abiy's office has accused Amhara's regional security chief, Brig-Gen Asaminew Tsige, of being behind the coup plot.
His whereabouts are unknown, and he is on the run, Amhara regional deputy head of security Gedebe Hailu told the BBC.
Brig-Gen Asaminew was among a group of high-ranking military officers released from prison early last year when the previous government moved to free political detainees in response to public pressure.
The general had been in custody for nine years for allegedly plotting a coup.
According to Reuters news agency, Amhara's top officials had convened a meeting on Saturday to discuss attempts by the general to recruit ethnic militias.
Brig-Gen Asaminew openly advised the Amhara people this month to arm themselves, in a video spread on Facebook and seen by a Reuters reporter.
What do we know about the attacks?
The chief of staff of the Ethiopian army, Gen Seare Mekonnen, was killed on Saturday evening by his bodyguard, who is now in police custody, the prime minister's press office said.
He was at his residence along with another general, Gezai Abera, who was killed, it added.
The government said it had reason to think the attack was linked to the assassination of the governor of Amhara, Ambachew Mekonnen, a few hours earlier in Bahir Dar.
Mr Ambachew was killed at a meeting in his office along with his senior adviser, Ezez Wasie, while the region's attorney general was wounded.
Lake Ayalew has now been appointed as the region's acting governor.
Many of those involved in the coup attempt are under arrest and operations are in progress to detain others, the PM's press office said.
"The coup attempt in Amhara regional state is against the constitution and is intended to scupper the hard-won peace of the region," it added.
"This illegal attempt should be condemned by all Ethiopians and the federal government has full capacity to overpower this armed group."
A toxic political atmosphere
By Emmanuel Igunza, BBC Africa, Nairobi
These are tumultuous time for Ethiopia and Prime Minister Abiy, who is already facing increased ethnic tensions.
Chief of staff Seare Mekonnen had only served as head of the military for a year having being appointed by Mr Abiy, who made sweeping changes in the security apparatus when he took office last April.
It is clear there is still significant opposition within the military opposed to the PM's style of leadership.
The killing of Amhara's governor is also a big blow for Mr Abiy, who is credited with installing Ambachew Mekonnen in office.
He was a key ally in Amhara, which is itself facing security problems and clamour from some groups for greater autonomy from the central government.
The first general election since Mr Abiy came to power is supposed to be held next year, but it is very hard to see how this will go ahead in a country that is highly polarised. The atmosphere is just too toxic.
Why is Amhara so important?
The homeland of the Amhara ethnic group is the country's second most populous region and has given Ethiopia its state language, Amharic.
Violence between the Amhara and Gumuz ethnic groups left dozens of people dead last month in Amhara and its neighbouring region, Benishangul Gumuz.
Ethnic violence, typically sparked by land disputes, has displaced nearly three million people across Ethiopia.
Another issue the prime minister is having to grapple with is unrest within the military.
In October, he said hundreds of soldiers who marched to his office to demand a pay rise had wanted to kill him.
which killed two people and left more than 100 injured.