Baghdad, Iraq ? A group led by a Jordanian militant, Abu Musab al-Zarqawi, said Tuesday that it had executed a second American hostage in as many days and threatened to kill a British man who was abducted along with the two Americans last week.

A statement posted on an Islamic Web site by the Monotheism and Jihad group asserted that it had killed Jack Hensley, 48, a civil engineer from Marietta, Ga., and would soon post a video of the slaying on the site. However, the video had not been posted as of 3:30 a.m. local time today.

On Monday, the same group posted a video of what it said was the beheading of Eugene “Jack” Armstrong, 52, a Michigan native.

The group said Tuesday that it intended to kill Kenneth Bigley, 62, a British engineer, unless authorities released all Muslim women from two U.S.-run prisons in Iraq, Abu Ghraib outside Baghdad and Camp Bucca in southern Iraq. The U.S. military says only two women are being held by U.S.-led forces and neither is confined at those prisons.

An estimated 300 people have been killed and hundreds injured here during the past two weeks, as a deteriorating security situation has hampered reconstruction efforts and threatened to derail the process of establishing democratic rule in Iraq.

Two U.S. Marines were killed Tuesday in separate attacks in the western province of Anbar, and four soldiers were injured when a car bomb exploded near a convoy on the perilous road to Baghdad International Airport, the U.S. military reported.

U.S. forces have set a $25 million reward for information leading to Zarqawi’s capture and staged repeated airstrikes on Fallujah, the suspected base of his operations.

Hensley, Armstrong and Bigley all worked as civil engineers for Gulf Supplies and Commercial Services Co., a construction company based in the United Arab Emirates. They were helping to renovate the Taji military base north of Baghdad, which is used by the reconstituted Iraqi army.

The three men shared a house in south-central Baghdad. Neighbors said as many as 10 gunmen barged into the house Thursday and abducted the men. Two security guards who normally protected the house were not present that day, the neighbors said.