Days after he was denied access to a federal shelter for child migrants, Sen. Jeff Merkley, D-Oregon, is demanding answers from the Office of Refugee Resettlement about the roughly 11,000 children under the care of the federal agency.

"Under U.S. Attorney General Jeff Sessions' 'zero tolerance' policy on immigration prosecutions, children must be forcibly separated from their families, falsely labeled 'unaccompanied alien children,' and transferred to your agency's custody," Merkley said in his letter to Office of Refugee Resettlement Director Scott Lloyd.

"According to press reports, more than 600 children have already been separated from their families since the implementation of this policy at the beginning of May, bringing the total under your care to a staggering 11,200."

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Merkley drew headlines Sunday when he posted a Facebook live video of his attempt to access a migrant children's shelter in Brownsville, Texas. He was first denied entry, and then ultimately confronted by police. Federal officials said Merkley hadn't given enough notice.

Merkley's letter comes two days after similar demands were made by Democratic Sen. Bob Menendez of New Jersey, whose staff were barred from reviewing immigrant processing facilities near San Diego. Menendez called Sessions' immigration policies "cruel and un-American."

Merkley's letter lays out 19 questions and requests a response by June 15. Merkley wants information about the age of the children, the condition of the facilities, and more.

See the letter and all 19 questions.

Similar questions posed by CBS News to the Office of Refugee Resettlement and the United States Department of Homeland Security have not yet been answered. The Administration for Children and Families, the agency that oversees the Office of Refugee Resettlement, said it "does not comment publicly on congressional requests before providing an official response to Congress."

In a statement to CBS News Wednesday, before Merkley's letter was sent, Southwest Key Programs, the non-profit contracted to manage the Brownsville facility, described in broad terms the services it provides to children housed there.

"We provide round-the-clock services including: food, shelter, medical and mental health care, clothing, educational support, supervision, and reunification support," the organization said. "We regret having to turn away Senator Merkley at our Casa Padre shelter. The U.S. Office of Refugee Resettlement prohibits any facility from allowing visits that have not been approved by them, even if it is a U.S. Senator."

Read Merkley's 19 questions below:

1. Since the "zero tolerance" policy officially went into effect on May 7, 2018, how many children have been transferred to the Office of Refugee Resettlement (ORR) or its contracting agencies' custody? How many of these children were separated from their parents and how many entered the United States as unaccompanied minors?

2. How many children in the following age groups is the ORR or its contracting agencies currently holding in detention?

a. Newborns to 1 years old?

b. Between the ages of 2 to 4 years old?

c. Between the ages of 5 and 8 years old?

d. Between the ages of 9 and 13 years old?

e. Between the ages of 14 and 18 years old?

3. What is the average and median length of stay for these children? How does this figure compare with 2017 and 2016?

4. How many ORR facilities are currently operational for children and what is the capacity for each of them?

5. How many organizations and companies contract with ORR to house children separated from their parents and unaccompanied minors? What are their names?

6. What bathroom facilities and accommodations do these shelters have and are the conditions sanitary? For instance, are there bathrooms, showers, and towels? Are there rooms for the children that have beds with bedding? If so, how many beds per room?

7. What are ORR's mandated facility standards for shelter contractors?

8. What are the required staffing ratios, broken down by age groups?

9. What categories of staff require credentials to work in these facilities?

a. Which credential requirements are ORR imposed and which are state required?

10. How many facilities have state monitors? How often are facilities monitored?

11. What is the process for providing education and schooling to children? How many days a week is schooling provided and how many hours per day?

12. Are these children permitted to leave the premise to go outside? Is there recreational time available for physical exercise?

13. What is the process for providing health care services to children? In order to receive medical care, are health care providers on-site or are ill children transported to hospitals or other health care providers?

14. What services are provided to children for trauma support and mental health care? Are mental health care professionals providers on site?

15. Are children informed, in writing or verbally, that any information they provide to medical professionals or facility staff will be added to their immigration record and can be used against them in immigration court?

16. Please provide a current breakdown by category of children's approved sponsors.

a. Category 1 – Parents

b. Category 2 – Family members

c. Category 3 – Non-relative

17. How do detained children communicate with their parents? How often are they allowed to communicate with their parents, through what method, and for what duration?

18. Are in-person visits between the children and their parents permitted? If so, what are the procedures and policies surrounding visitation and how often do they occur?

19. Are in-person visits between the children and their lawyers permitted? If so, what are the procedures and policies surrounding visitation and how often do they occur? Under what circumstances, if any, would a lawyer be denied entry to visit with a child?