SSH-ing into EC2 using Lambda Function (Authentication failed, Authentication Error)

I am attempting to SSH into an EC2 instance through a Lambda function using the Paramiko library. Despite double-checking the key file, username, security group permissions, and ensuring that everything is in order, I am still encountering an Authentication Error.

import json
import boto3
import paramiko

def lambda_handler(event, context):
    # boto3 client
    client = boto3.client("ec2")

    # getting instance information
    describeInstance = client.describe_instances()

    hostPublicIP = []
    # fetchin public IP address of the running instances
    for i in describeInstance["Reservations"]:
        for instance in i["Instances"]:
            if instance["State"]["Name"] == "running":
                hostPublicIP.append(instance["PublicIpAddress"])

    print("Active EC2 instance IP: ",hostPublicIP)
   
    key = paramiko.RSAKey.from_private_key_file('EC2-Key.pem')
    print("RSA Key: ", key)
    
    host = hostPublicIP[0]
    print("Connecting to : " + host)
    ssh_client = paramiko.SSHClient()
    ssh_client.set_missing_host_key_policy(paramiko.AutoAddPolicy())
    
    ssh_client.connect(hostname=host, username="ec2-user", pkey=key)
    print("Connected to: " + host)

    return {"statusCode": 200, "body": json.dumps("Thanks!")}

Error:

Response
{
  "errorMessage": "Authentication failed.",
  "errorType": "AuthenticationException",
  "stackTrace": [
    "  File \"/var/task/lambda_function.py\", line 40, in lambda_handler\n    ssh_client.connect(hostname=host, username=\"ec2-user\", pkey=key)\n",
    "  File \"/opt/python/lib/python3.8/site-packages/paramiko/client.py\", line 435, in connect\n    self._auth(\n",
    "  File \"/opt/python/lib/python3.8/site-packages/paramiko/client.py\", line 764, in _auth\n    raise saved_exception\n",
    "  File \"/opt/python/lib/python3.8/site-packages/paramiko/client.py\", line 664, in _auth\n    self._transport.auth_publickey(username, pkey)\n",
    "  File \"/opt/python/lib/python3.8/site-packages/paramiko/transport.py\", line 1580, in auth_publickey\n    return self.auth_handler.wait_for_response(my_event)\n",
    "  File \"/opt/python/lib/python3.8/site-packages/paramiko/auth_handler.py\", line 250, in wait_for_response\n    raise e\n"
  ]
}