At least 23 people are reported to have been killed by suicide bombers on motorcycles who simultaneously attacked two buildings including the national police academy in Chad's capital on Monday.
The attack is the first of its kind since Islamic militants threatened the country earlier this year.
There was no immediate claim of responsibility for the attack in N'Djamena but suspicion fell on Boko Haram, the jihadist group based in neighboring Nigeria that already has attacked Chadian villages along the lake that divides the two countries.
— annadjibZer (@annadjib) June 15, 2015
Chad has been a major military ally in the fight against the insurgent group, and Boko Haram's leader has publicly threatened the Chadian president in retaliation.
Witnesses said a suicide bomber on a motorcycle blew himself up outside a government building where the head of the national police is based after security officers unsuccessfully tried to subdue the attacker by shooting at him.
A police official who spoke on condition of anonymity as he was not authorized to speak to the media said at least 10 people had been killed in that attack.
A second explosion also went off at the headquarters of the national police academy in the capital at the same time, witnesses said.
[ooyalacontent_id="FveGJvdDrDret__i8XtSBqqCklF2P3PH"player_id="YjMwNmI4YjU2MGM5ZWRjMzRmMjljMjc5" auto_play="1" skip_ads="0"]
An international security source told Reuters seven people were killed in the initial suicide attack and another 35 people died when explosives planted at the police training school went off. A second source said at least 20 people had been killed but did not give a breakdown of the incidents.
The Chadian information ministry told AP 23 people had been killed along with four attackers.
The area has since been locked down, though Chad's Interior Minister Abderahim Bireme Hamid told Reuters that the situation was ongoing.
Back in February, Boko Haram's leader taunted the Chadian president.
"You sent 7,000 of your soldiers. Why didn't you send 7 million? This is few… 7,000? By Allah, it is small. We can capture them in an attempt or two," Abubakar Shekau said, according to a transcript later released by SITE Intelligence Group.
The same month, Boko Haram carried out its first attack on Chad's territory — killing seven people in the border town of Ngouboua. The militant group has also carried out attacks in Cameroon and Niger.
The Associated Press contributed to this report.