Senator Bob Menendez says senior staff members from his office were denied the ability to review conditions at immigrant processing facilities near San Diego. Menendez, a Democrat from New Jersey, made the claim in a letter released Tuesday to Kirstjen Nielsen, Secretary of the Department of Homeland Security.

Menendez said the staffers were planning a visit to the San Diego-Tijuana border on Saturday and requested access to the sites nine days in advance. Menendez wrote that the staff were told nine days was not enough lead time to prepare for such a visit.

"This deeply disturbing decision appears to be an outright attempt to undermine congressional oversight of the executive branch. This is unacceptable," Menendez said in the letter. Menendez said previous visits to border facilities have not required as much advance notice.

A Department of Homeland Security official disputed Menendez' timeline. The official acknowledged that the staffers inquired about the visit on May 24, but said they didn't respond to an email sent the next day asking for more details until May 29.

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In response, a Senate Foreign Relations staffer told CBS News that staff called DHS to give more details on May 25, eight days before the planned visit.

Menendez's letter to Nielsen requests an explanation -- and a promise that the staff will be allowed access in the future.

"I urge you to reverse DHS's troubling decision to refuse congressional oversight and work with my office to ensure that staff will be granted access to CBP facilities in the next month," the letter says. "The Administration's new policies are cruel and un-American, and U.S. citizens deserve answers from their government."

Menendez first revealed the failed attempt by senate staffers via tweet on Monday, in response to a video showing Senator Jeff Merkley, a Democrat from Oregon, being turned away by police Sunday while attempting to gain access to a juvenile immigration detention facility in Brownsville, Texas.

DHS said Merkley also did not give enough advance notice, and noted that he was recording video, which the department said could have violated the privacy rights of the children.

On Tuesday, Merkley described difficult conditions he witnessed inside a different Texas facility, the McAllen Border Patrol Processing Center.

The scrutiny of conditions in immigration facilities has intensified since a May 7 announcement that the Justice Department would seek to charge every person caught crossing U.S. borders without permission. The new policy sparked heated debate when it was revealed that children would be separated from their parents at the border.

Attorney General Jeff Sessions defended the new policy in an interview Tuesday. "If people don't want to be separated from their children, they should not bring them with them," Sessions said.