Sayed Ahmad Safi, the provincial governor's spokesman, confirmed that Laghmani, who is the deputy chief of the National Directorate for Security (NDS), was among the dead.
James Bays, Al Jazeera's correspondent in Kabul, said: "Behind the scenes Laghmani was key ... his death will be a big blow to the Afghan government and their fight against the Taliban."
Several other government officials were also thought to have been killed.
Targeted attack
The Taliban claimed responsibility for the attack, saying it had targeted senior officials.
The attack came as a group of government officials were inaugurating a mosque in the city of Mehterlam, 100km east of Kabul, Afghanistan's capital.
Lutfullah Mashal, the governor of Laghman, who witnessed the attack, told Al Jazeera: "The deputy chief of the NDS is from Laghman province.
"He came to Laghman this morning to participate in the reconstruction of the central mosque. He wanted to visit the mosque.
"But before entering the mosque the suicide attacker rammed into his vehicle and exploded himself in a big crowd of religious scholars and tribal elders."
An Associated Press photographer at the site said US troops and Afghan officials had surrounded the blast site.
The NDS is headed by an ethnic Tajik, and analysts warn that the killing of Laghmani, a Pashtun, could further exacerbate ethnic tensions in the country.
Hamid Karzai, the Afghan president, condemned the incident and the United Nations in Afghanistan said the attack was "indefensible" during Ramadan, the Muslim holy month of fasting.