The suspect, who also shot and wounded the policeman whose gun he took, later died in hospital. The ministry did not specify whether he had been shot by police or taken his own life.

The announcement came shortly after the United Nations confirmed a special envoy would be visiting Baghdad on Sunday to make preliminary findings on security after last weekend's bombings and a similar attack against government offices in August.

"One of our police officers was killed by a suspect involved in Sunday's bombings," a ministry statement said, adding that the incident occurred at 1:00 a.m. on Friday (2200 GMT on Thursday) at the Criminal Investigations Directorate in central Baghdad.

During the interrogation, a policeman offered the suspect water. The suspect then stole the policeman's gun and shot and wounded him before firing on Major Arkham Hachim with the same gun, killing him instantly.

The suspect was then shot — the statement did not say by whom -- and transferred to a nearby hospital, where he died of his wounds.

The interior ministry said it had recognized that "negligence led to this incident" and announced that it would name a judge to lead an independent inquiry.

It added that an "administrative investigation" would be carried out "to inspect the circumstances surrounding the incident, and who was negligent in their performance."

Sunday's twin suicide bombings against government buildings in central Baghdad killed 153 people and wounded more than 500, the deadliest attacks in Iraq in more than two years.

They followed similar attacks on government ministries in the capital in August which killed around 100 people.

UN Assistant Secretary General Oscar Fernandez-Taranco's trip to Baghdad, which will include talks with Foreign Minister Hoshyar Zebari, comes after intense lobbying by Iraq for an independent probe into the massive attacks.

But a diplomatic source in Baghdad told AFP that Fernandez-Taranco's visit was a "preliminary mission" and not the start of an inquiry into the violence, as requested by the Iraqi government.

"Oscar Fernandez-Taranco's program is not yet finalized, but he will certainly meet officials in the foreign ministry and other people necessary to the mission," Said Arikat, spokesman for the U.N. mission in Baghdad, told AFP.

Asked whether he would meet the envoy, Zebari said: "Yes, definitely, I am looking forward to meeting him."

U.N. Secretary General Ban Ki-moon told a news conference in New York last week that he would send Fernandez-Taranco "in response to a request from the government of Iraq."

Zebari said this month that the U.N. Security Council would appoint an investigator in response to a call by Iraq for an independent probe into the August bombings.

Baghdad has accused neighboring Syria of harboring the masterminds behind those attacks, and Prime Minister Nuri al-Maliki alleges that 90 percent of foreign militants who infiltrate Iraq do so via Syria.