BAGHDAD, Iraq — One bomber penetrated the secure compound of Iraq's most celebrated police commando unit. Another slipped into a mess hall where scores of Iraqi soldiers were eating.

Neither of the suicide attackers aroused suspicion for a good reason: Both were Iraqi security officers themselves.

Nearly 30 soldiers and police were killed and dozens more injured in the two bombings this month. The attacks at the headquarters of the elite Wolf Brigade in Baghdad and at an Iraqi army base north of the capital highlighted the challenge Iraq is facing from insurgents.

Infiltration was a specialty of Saddam Hussein's security apparatus and officials think many of the current cases are being directed by former regime elements.

Authorities also suspect that insiders are providing identities of police and military commanders, who are being gunned down on almost a daily basis, typically on their way home or to work. The interior minister, Byan Jabr, has alleged that the names and addresses of police are even being sold on the streets, the motive often profit rather than ideology.

About 1,600 "ghost names" have been discovered on the Interior payroll, Iraqi officials said, and the search is continuing.

"I think it [infiltration] is the greatest long-term threat to the security of the country," said a senior U.S. military officer, who declined to be named.

Especially disquieting, officials say, is that the attackers at both Iraqi bases were themselves Iraqis — not foreign jihadis, as most suicide bombers are thought to be.

On June 15, an Iraqi soldier sat down with 100 colleagues in a cafeteria on a base in Khalis, northeast of Baghdad, colleagues said. He apparently was wearing a belt rigged with explosives.

"The next thing we heard was a loud explosion," said one of the survivors, who declined to be named. "It was like a tornado that swept the place."

The bombing killed 26 Iraqi soldiers and injured 38. Some later accounts indicated that the infiltrator may have been a contract worker dressed as an army man. But several fellow soldiers indicated he was one of their own.

In the Wolf Brigade attack, three commandos were killed when the infiltrator detonated his bomb in their compound.

Infiltrators also pose a very real danger for U.S. troops who work alongside their Iraqi counterparts. The Pentagon has yet to release its findings in the suicide bombing last Dec. 21 at a U.S. Army mess hall in northern Iraq that left 22 dead. The attacker was wearing an Iraqi military uniform, officials say, but Arab news reports indicate the assailant may have been a Saudi impostor.

The implications of moles in security posts extend beyond bombings. There is great concern about insiders, acting for money or ideology, tipping off insurgents. U.S. commanders often privately wonder if someone was alerted in advance in the frequent cases where they and Iraqi allies arrive at raid sites only to find that all the suspects have vanished.

"There has to be a security violation with all the officers and policemen who are being assassinated," said Nori Jabir al-Nori, the recently appointed inspector general for the Interior Ministry, which oversees internal security. "They know when they leave, and they know when they come. This has to be an inside job."

Part of the problem, officials say, is a rushed hiring process that has allowed questionable applicants to join the police and military, sometimes outside of normal recruiting procedures. The previous interior minister added tens of thousands of employees, officials say, and hundreds have disappeared — with their security clearances intact.

Iraqi and U.S. security officials say screening procedures for new lawmen and troops are being improved through fingerprint checks of new applicants, database examinations and even interviews with friends.

But the drumbeat of bombings and shootings demonstrates that the updated procedures are far from foolproof. Relatively few records are computerized. And infiltrators may have no criminal records to speak of, just a willingness to work for insurgents in a country where allegiance to tribe and family often trump all other commitments.