A senator placed a hold Thursday on the nomination of Julie L. Myers to head the Immigration and Customs Enforcement bureau, the latest fallout from a controversial incident at a Halloween party.

Myers’ has drawn fire for an office party in which a co-worker appeared in prison garb and a dreadlock wig and darkened his face to appear African-American.

But Myers, in a letter to Claire McCaskill, D-Mo., who placed the hold, said she did not know that the co-worker had darkened his face with makeup.

McCaskill sent Myers a letter Nov. 6 asking her why the costume was allegedly praised at the party for originality.

“As you know, I already have serious concerns about your nomination,” McCaskill said. “Given the nature of the functions performed by ICE, this incident strongly undermines the claim that you can effectively lead this organization.”

Myers responded Thursday evening by e-mail, saying she “regretted” the incident. “I was surprised and mortified to learn the day after the event that the employee’s true skin color was not as it had appeared,” she said.

Photos taken of Myers and the employee were deleted. In her e-mail message, Myers acknowledged that she had asked to have the photos deleted because she realized in retrospect that a prisoner costume was harmful to “the agency’s goal of treating everyone in our custody with dignity and respect.”

Myers also provided a list of actions she has taken to promote diversity within ICE, in a letter she sent Thursday to Bennie Thompson, D-Miss., chairman of the House Homeland Security Committee.

“I can assure you that ICE fully embraces and supports [equal employment opportunity] and diversity as a core business principle,” Myers wrote.

Adrianne Marsh, McCaskill’s communications director, said the senator is not completely satisfied with Myers’ response.

Marsh also said a handful of senators, including Majority Leader Harry Reid, D-Nev., and Joseph I. Lieberman, I-Conn., chairman of Homeland Security and Governmental Affairs, expressed concerns about the incident.

McCaskill has no plans to lift the hold until she has time to discuss Myers’ response with her colleagues, Marsh said.

McCaskill had opposed Myers when the Homeland Security panel approved her nomination, 8-1, on Sept. 26.

McCaskill had criticized Myers for not providing requested data on the number of employers sent to jail for hiring illegal immigrants.

President Bush gave Myers a recess appointment in January 2006, after his first attempt to nominate her met stiff opposition. He resubmitted her name for consideration a year later.